I guess he can only hold it for so long.
He goes in the pull up just fine. He holds his pee and poop when in underware.
I like the therapists approach, talk to him and say what you are going to do, the pullups are enabling him to do it in them. Once he is out of them he will not like to make a mess continually, so it may take a couple of days for him to get it. Mayeb also take a step back and let him get on with controlling his body himself sometimes the more that is made of it the more it becomes. Ignore what is going on give him the pullups in the morning or lay them out with his clothes the night before and say nothing, if there are accidents thats okay, no scolding as that will only make him more anxious, Good Luck..