I feel your pain, because we've had our struggles even getting DD to just sit on the potty, never mind do anything in it! She's 3 years and 8 months already and has wanted nothing to do with the potty. She is also very bright but very strong-willed and stubborn, and no amount of incentives was getting her to change her mind. Even seeing other kids in preschool use the potty didn't motivate her. She just seemed determined to stay in diapers.
Past few months I did start telling her that if she wanted to go to school after she turned 4, she would have to start using the potty because they didn't let them wear diapers to school (technically she isn't allowed now but the teacher lets it slide, since she does not expect all 3 year olds to be potty trained and it has not been an issue). Now that Sept is 5 months away, I figured it was time to get down to business. We've been going "cold turkey" during the day since yesterday and just letting her have a diaper for naps and bedtime. I don't expect her to be sleep-trained for a while, I just want her to start realizing what we now expect of her, which is basically to try to get to the potty and know what it feels like to be wet, which is her signal that she needs to sit on the potty.
So during the day she is in underpants and leggings (no snaps or buttons or zippers to fuddle with). She can drink all the juice she wants. Periodically we try to get her to sit on the potty, but she tends to say NO and I don't want it to turn into a power struggle. So if she gets wet, oh well. She has started trying to run to the potty once she realizes the pee is coming out, and we still have her sit for a minute even though she's emptied. She gets 1 M&M for trying, 2 for pee, and 3 for a poop (of course, neither yesterday nor today has she had any poop). Also, she gets a stamp on her hand and gets to put a stamp on her potty chart when she actually pees or poops on the potty, and of course, lots of high-fives and positive words. With accidents, we just say oh well, that's okay, lets try again next time (it's only day 2, mind you) and get dry clothes on her.
She's done some screaming and flipping out about having to wear underpants and having to try sitting on the potty, and at first, when we would try before, I would feel bad and back off and figure we would try again later. Now I'm just not listening to it anymore. I can't MAKE her go on the potty, but I don't have to keep putting her in diapers either and taking the underpants off just because she's having a hissy fit. Bit by bit, I think she is starting to come around. She isn't protesting so much about wearing the underpants because I think she is realizing that this is the new routine and no amount of screaming is going to change it (she can be a it of a drama queen and yes, I did try letting her pick out her own underpants and that still wasn't enough for her). I'm just standing firm and telling her that she is a big girl now and she needs to start trying to go on the potty like all the rest of her friends.
I know many people have told me, when I have posted on here myself about trying to potty train, that when they are ready, it will go really quickly and smoothly, without hardly any fight at all. But we've been waiting over a year for DD to make up her mind and I just decided that it was time to take control and be a bit more proactive. Physically, there really isn't any reason why she can't start going on the potty - she stays dry for hours (not always overnight, but again, that comes later), she knows what the potty is for, and she likes watching us go on it and even likes flushing it after (she herself is using her own little training potties, not the big grown-up potty). It's just overcoming this mental barrier that she has and getting her to realize that wearing panties and going on the potty is not the end of the world.
So far, she's gone pee twice on the potty - once yesterday and once today. Both times were when I figured she needed to go (because it had been a couple of hours and she had been drinking a lot) but I had to just lead her by the hand and make her sit on the potty. If I asked if she wanted to or needed to, the answer was always NO! But then as she is sitting on the potty, screaming like a banshee, here comes the pee! She can't keep holding it anymore and into the potty it goes! Then she's all excited and happy! She goes from screaming to giggling in an instant! And I tell her how proud I am, look what you did, good job! She gets her M&Ms and her stamps and we tell her we are going to keep working at it, because it takes practice (she has been asking if she can get her diaper back now that she's peed on the potty, because I think she is thinking it's a one-time thing and now she's all set, but again, standing firm on only having the diaper on for bed).
As for your son, I don't know if I would let one set-back today make you think he isn't ready yet. It may take many many accidents and rewards for going in the potty before he gets the concept. There are physical signs of being ready that I would be familiar with to make sure you are not expecting too much of him. You may have to just "cold turkey" it like what we are trying to do. Some may not agree with what I am doing with my DD but my feeling right now is that as long as she has the option to go in a diaper, that is what she will continue to do - once she has no choice but to go on the potty, she'll get the picture, with a little reward thrown in to help her along.
Here are the posts I wrote about trying to potty train her, if you find any of them helpful:
http://www.mamapedia.com/questions/5304277468700606465
http://www.mamapedia.com/questions/8665888092856647681
http://www.mamapedia.com/questions/17376182349426982913