Just two cents here... Would signing help as opposed to verbal? I've heard kids can clue into signing easier than verbal at times.
Also, if you need him to pee, is he a good drinker? I've encouraged a child in training to drink a fair bit (fave juice watered down) and then I set a timer for myself of about 45 min as that's how long takes for a cup or so to start putting pressure on bladder (remember baby ultrasound days!?). At the timer, I'd watch for any sign of discomfort, fidgeting, and I'd talk about feeling so much more comfortable with an empty bladder. I'd pointed to where his bladder was just below his tummy. Did over the course of many days as we can't get them to drink too much! He's lucky with all the effort and research you're doing; going above and beyond for him.