I have a daughter who is now 5, and a son who is 20 months. Even though my daughter is not a teen, she has things, and eats things, her brother is not yet allowed to. And he notices and sometimes has a fit over it.
My two couldn't be more different personality wise. My daughter is a full on girly-girl and my son is already a rough and tumble sports fiend, even though we are far, far from a sports family. And while my daughter was very defiant and often tested our limits, she would realize defeat sooner. My son is not as defiant as he is stubborn. He often listens to us and likes to help, but he'll get in a tizzy over something (not playing a game he likes on Wii) and not let up.
Yet, they both responded to time-outs. The thing about time outs is that YOU MUST BE CONSISTENT. That means giving him a time out EVERY time he does not respond to your asking him to stop doing something, or every time he does something he knows he shouldn't (like pulling hair or biting).
And it is best if the time outs occur in the same place when you are at home. We use a pack n play. When out shopping or dining or visiting friends, CONTINUE to give him time outs when needed. If he thinks he can get away with stuff when other "outsiders" are looking on, he will take advantage of it.
I took my daughter outside when we were at restaurants, so she wouldn't disrupt other diners with her crying and yelling. It was sometimes a total pain in the butt, and people coming in would sometimes look at us oddly, but IT WORKED! She is a shining example of a well behaved child when we eat out and, because of that, she gets dessert, since we're not trying to rush out as fast as possible.
We make our son sit down when he's in the pack n play, because he seems to calm down faster than he does when standing up. And since our pack n play is in the family room, we make him face the opposite direction of the TV (when it's on) or turn it off.
And I can't tell you how great is is for me to be able to put my son in the pack n play (a safe place) and walk into another room when I also need a "time out". Cooling off is good for the parent, too.
If he's screaming too loud during the time out, we tell him, "No screaming. If you don't stop screaming, you'll stay in your time out." (this works).
And you must make it clear that you are not punishing him for wanting what his brother has (it's only natural) but for the screaming, because that is unacceptable behavior. We made it clear that screaming is only for "emergencies" (which there are plenty of with 2 kids anyway). When my daughter was about 2 and a half, I explained that if she screamed every time she didn't get her way, I might learn to block it out and then when she really needed help, I wouldn't realize it. That made a lot of sense to her.
So, you say, "No screaming." If he continues, say, "I said, 'No screaming'. If you don't stop, you'll have a time out." If he doesn't stop, then put him in a time out. After a month or so, stop warning him. Just tell him, "Time out for screaming!" as soon as he starts.
And, during the time out, crying is okay (they are being punished after all), but screaming after the first 20 seconds is not. So, you make sure they understand that the longer they scream, the longer the time out. I used to say, "I still hear screaming, so you have to stay there longer." and I would totally ignore them while they screamed. I would give them no reaction (since that's what they want). Eventually, they both gave up screaming after the first 30 seconds, which I let them have to "get it out of their system".
My daughter is so quiet, now, she sometimes has to remind me she's in a time out... I'll hear her say (from her time out spot in the living room), "Mom! I'm not crying or screaming."
And I say all this like it's nothing, but I know it's really hard. I can't tell you how horrible I felt doing this with my daughter at first. She didn't seem to be getting it (we started when she was 14 months).
However, I stuck in there and it paid off better than I could have imagined. My daughter went from 5 - 10 time outs a day (did I mention she was a biter?), to 1 or 2 a week in the course of 3 months. Now, she might get one once a month.
And just to be clear, we are not mean or overbearing parents. This is just what we do when our kids get out of hand or do things that are not acceptable. Both my son and daughter are the happiest kids I know and we do a lot with them. My daughter (who has benefited from this longer) has tons of friends and their parents love having her over. I enjoy her company more than I ever imagined I would and her teachers (she's in preschool) say she is the kindest and most reasonable child they've ever taught.
My son took a couple months longer to get the hang of it, and is still a work in progress, but about a month ago ago we had a breakthrough. Now, when we say, "Do you want a time out?", he almost always immediately stops doing whatever it is we don't want him to do (like putting things in the DVD drive). My husband's friend (who is also a dad) recently witnessed this happen and was blown away.
And when he does go too far, he now runs and climbs into his pack n play and sits down, not facing the TV , when we say "Go have a time out." In fact, when a friend of mine was disciplining her daughter with a time out while visiting, my son thought she was talking to him, and ran and climbed into his pack n play! Of course, I got him out and made sure he understood he had done nothing wrong, but my friend could not believe it.
I know not all children are the same, but the same can be said of parents. Some (like many friends of mine) gave up too soon or were simply not consistent. It's worth the pay off if you keep it up. You'll be happier because there will be less conflict and your kids will be happier with the consistency, knowing what the limits are, and what will happen if they go too far. Punishment is not really about controlling your kids as much as it is a way of showing you love them (when done correctly). If you don't care enough to help them learn about acceptable behavior, what kind of person will they end up being?
You're on the right track... just keep working on it! And make sure your husband does the same!