M.S.
I feel like he is being set up to fail by waiting till he started to go. Especially if he has to go bad, good chance it will all come out before he makes the toilet, not to mention - you have to be right by his side to catch the first sign that he is about to go. Why not just set up a bathroom routine. Don't ASK if he needs to go, give him two choices... "Do you want to go potty in your bathroom or my bathroom?" or "Do you want me to read you this book or that book while you go potty?" And offer this up every hour or so - depends on how often he needs to go but no more than two hours. If he is ready - this will happen quickly - if not then it could be months of work. My son was 3 and my daughter was almost 4 but since they were ready, we only had a couple of accidents. And of course, be patient and keep it low stress (sounds like you are doing a good job of this already).
You don't want him holding his poop - that could lead to something bigger (major constipation - stretched colon - encopresis - years and years of getting things back on track) - NOT FUN!
Hope that helps!