Son Refuses to Go #2, I'm at Wits End!!

One of the best tricks is have the Dr talk to him about how dangerous it is for him to hold it!! That can reek havoc on his system.
Our Dr telling my kids something is a whole different thing then me saying something.
He knows how to go, obviously, my suggestion is be matter of fact about it but all big boy priviledges cease until he goes on the potty. Buy a fun potty seat and try to encourage him but don't get angry about it. He knows what he is suppose to do.
Take away big boy toys, all that stuff until he shows he is a big boy. Boys in general can take longer in potty training.

Put him back into pullups to save you both in the sanity dept and let him "earn" back underwear. Let him cry about sitting on the potty, but tell him nicely "you know until you can show me you can do this yourself, I need to set you on the potty for a minute. If you don't have to go that is fine but sit here for one minute". Set a timer.
Don't scold, try to keep your cool and just be whatever about it.
He will do it when he is ready and him holding it can really be dangerous.
Have a friend or family member call into your home and pretend to be his favorite super hero or character (or even Santa) praising him for heading to be a big boy and with the prospect of big boy reward if he starts going on the potty himself.
Just a suggestion. Each child is different and some go at their own pace. Hang in there! :)

Activia yogurt every day, even Metamucil every day... Make it so he "can't" hold it in.

My son is almost 4 and still in a diaper. Yesterday I started having him change it himself. I took the sides apart and let it fall and told him to put his poop in the toilet, and I gave him toilet paper and told him to clean the piece off the floor. I gave him more paper and told him to wipe his bum. He was so excited after. Now it is his job and he is starting to learn. I think trying to force it is going to be harder, obviously it is so traumatic for him he is actually hurting himself. Let him do it himself, I think when he finally does it it will give him a great sense of control and he will know that you will not push him to do things he's not ready for yet. In all other ways he may be ready, but mentally he's got a roadblock to get through.

My pediatrician said boys take longer and it's normal for them to just begin getting it at 3.5 It's way common for them to struggle with #2. I used to just have my daughter tell me when she had to go I'd ask her if she wanted to go on the potty and if not, we'd put a diaper on her just for that. I had to be really patient, but something seems to just click and they want to do it on their own. I also motivated her with a really big treat if she did it, and then a big reward if she filled a sticker chart just doing #2. She was so happy and got it after that.