Q.R.
Ok...I'm not a doctor but....My son had this same problem. It came from constipation. He was 8 years old and he leaked all the time. One day he changed into his swimming trunks and was walking to the pool and I told him he needed to change his shorts. He cried why these are clean and then looked down and was so embarassed. He really had no idea it was happening. He had to take Miralax once a day and it really worked. The problem is it takes weeks maybe even months to get the muscles down there retrained to work again.
I would talk to a doctor. How can you possibly potty train him if he doesn't have control? You really can't. He'll just end up super frustrated and so will you.
See a doctor :-) Good luck.