I have been working for a year to train my4.5 year old autistic step son. We had almost no success until just over a month ago when I figured out a new potty routine for him he now goes on the potty and he is proud of it and has fun doing so. This is what is working for us.
#1 we drew a picture of a potty and put it in the bathroom, everytime he has a potty success he puts a sticker on the potty and gets a sticker for himself.
#2 to get him to sit and stay on the potty we give him a sucker and keep some toys around his potty.
#3 when he goes we really make a big deal out of it and if he does not go we just tell him that it is ok or we don't say a word.
He has success on the potty now about five times daily and I believe that the sticker reward is what really got him going. He had something to shoot for.
You know your son best, but I would do the "tricks" that you use with potty training any age. First if you think he can handle being wet without having problems put him in underwear. Next I would probably try having him sit every 30 min (I suggest sitting or you have to potty train twice once for pee and once for poop). Then as he has fewer accidents or picks up on the idea I would lengthen the time between sittings. A potty/sticker chart would be a good helper or just a reward for each sitting (just for sitting and trying at first then for actually going. It will most likely be a tougher road for you, but it will all work out in the end!