I agree don't yell. Many kids use potty as a way to assert independance and challenge their mom. Believe me, they know what they're doing, and they know it's freaking you out. I'm most certain since you say he has mastered both skills but has regressed, this is more an issue of him finally discovering he has a power that mom can't control.
Here's what to do:
Get a visible rewards chart. Either purchase an already made one (Check out Melissa and Doug products or Target which sells a really niceMelissa/Doug type wood/magnet responsibility chart that can be easily adapted. Or make one yourself using his favorite t.v. or book character stickers. Put down a section for each skill needed for good potty habits. You know 1. Remembering to go sit on the potty. (even if they don't "go" they should be rewarded for the effort). 2. "Making a rock" as my boys love to call it,if they actually get the job done. 3. One for pee. 4. Using wipes or toilet paper. 5. Flushing. 6. Washing hands with soap/water. 7. Going when he "feels" it, and not without mom reminding him.(this is real important to reward and reinforce!!)
Be sure to purchase a variety of prizes that you know he'll work for. Even have some "grand" prizes. I filled a small rubbermaid shoe storage box with matchbox cars, action figures, mini-play dough etc. Each time he would sucessfully complete a duty he'd get a sticker that could be applied toward a certain type of treat at the end of the day. You can even put together a simple point system that shows him what he'll get if he accomplishes his "tasks" well. Make it a fun game, that gives him a sense of control over himself rather than making it a war with you. Most of all, reinforce a sense of accomplishment by constantly telling what good progess he is making as the stickers/magnets start filling the board. If at the end of the week or when you feel the board is full, then you can offer him a choice of a "grand prize" something that appropriately rewards him for his good work. Be sure to tell him he "earned it" and that in life all hard work can result in good things such as knowing "he knows how to be a big boy and use the bathroom with no help, etc.
Even though he is already potty trained, be sure to periodically throughout the day, "remind" him to go to the potty, and follow him in just like before. If he bulks, remind him that he can still recieve a small reward at the end of the day for his efforts, and remind him that at the end of the month/week etc. he can work toward a special treat. Treats can also be other things like a special meal, or ice cream etc., but I found that something tangible was much more effective. They need to see what they're working for. I think this is a boy thing.
If you have a particularly willful little guy, put him on a timer. This was very effective with my youngest. We never used a reward chart with him. He didn't respond well. He's not really into things, but he is into gadgets and electronics. I found this fantastic little product at www.onesteapahead.com called the "POTTY WATCH". It looks like a child's watch, but is a timer that can be set by you for 30, 60, and 90 minute intervals. When the cute watch that looks like a potty plays a happy little tune (not annoying and just long enough to get em moving) it signals that its time to go sit on the potty. This is really great for those kids less than motivated to drop activities when nature calls (my little one was one of these). They can't resist and go nuts calling the troops to rally on toward the bathroom. Even his friends and whoever is near him that know what the watch is about want to go running to the bathroom! It's like the pied pieper or something. It is $9.95 and comes in blue and pink.
This method works well with a kitchen timer as well. Potty watch did not exist with my older kids, but I had read about using a timer to teach kids to use the potty. (This method is also good for kids that can't feel when they need to go) and I went to a gourmet cooking store where you can find the most interesting assortment of kitchen timers...cows, chickens, cars, flowers, etc. I let them pick what they wanted. My oldest picked a cow timer and we named it "pee pee cow". He loved it. I made him carry it around the house with him and would set it for 30 minute intervals and lengthened the intervals as he got better with using the potty on his own. Finally, he on his own stopped using it.
Also reinforce with videos and books. Some good ones are "Once upon a Potty" for boys both in book and video. Duke university also has an excellent program on video called "Potty Time", and don't forget "Elmo's Potty Time" and the Elmo doll sitting on his own potty. There's a great book at Target called "It's Potty Time" that flushes when a child presses an electronic button. I leave this in the bathroom with other potty related books to be read while getting the job done. They love it and tend to get the job done when they are busy reading.
This is a great time to teach him about the seriousness of germs and the importance of cleanliness. One of mine went through the play with the "pooh" stuff until he got to see pictures of microscopic germs in a children's science book and I made the connection between illnesses and germs and reinforced that's why we put it in the potty instead of make things out of it.
Last but not least, reevaluate your bathroom set up. If it's not kid friendly, it might be a deterrent. If your guy doesn't like his potty seat, or its too much work to get the potty set up for use on his own, he's not going to bother. I have all boys. I hate to say it, but they are not into work. I found out they didn't like having to put the potty seat on the potty, and didn't like dragging the stool the used to get on the potty from the potty to the sink to wash their hands. I found a cool potty seat that has three layers; a lid, an adult seat, and a child's seat all in one. All the kid has to do is open the toilet seat to his size and sit. I keep two stools in the bathroom now. One by the sink and one in front of the potty.
I know its alot, but I've been through alot with this department. I can happily say we've had success and are currently doing very well with our guy who just turned three a couple of weeks ago. He's almost there. He's been successfully making rocks for three days now. We started potty training about 3 weeks ago. Right away he mastered peeing. Boys love this! Especially if it can be a game like shoot the Cheerios. For my house, number two is equated to "constuction time". My boys loved pretending to be at the rock quarry and got a kick out of the idea of making rocks. Find what works for you! Good luck.