Potty Training Advice for #2

We are almost 2 months into potty training and my son (3 yrs old) has the pee pretty much down but refuses to 💩 on the potty. He will hold it till he can’t hold it any longer. He also knows he has to go just doesn’t seem to know how to release the muscles to let it out. Sometimes he says he has to 💩 and he will get on and off the potty multiple times within a 30 min window and then end up going in his underwear. I don’t know how to encourage him to (1) sit long enough to let it happen and (2) get him to relax. I’ve tried the ‘blowing up a balloon’ breathing thing and having him clean up his accident himself. Nothing seems to be helping. He’s also not constipated and eats fruit with every meal so his diet isn’t the issue. Any advise would be appreciated.