He sounds like my dd who is 5 in april !!
I have posted on here many times at my utter wits end with her. Especially leading up to starting school, as I was so worried about her pooing and weeing there and getting picked on.
I sought help from my gp a year ago, and she ruled out anything physical - she then advised star charts (as if we hadn't tried that one a million times!) but said we should actually physically taker her after every meal, and once inbetween.
This worked wonders for a while - then of course we got lapse, have busy lives and before we knew it back to square one.
Have now enlisted teachers help at school, as she as coming out wet and close to tears. They give her stickers and make sure she goes to loo. I do same at home. There really is nothing else you can do !
What I can say though is that I have one hundred percent STOPPED getting angry with her. I might sigh, say o dear, or what a pity, but not much else, I just tell her to get changed, or help her, and we start again. A few times a day she will go completely by herself to the loo, which is amazing ! But she still has lots of accidents too, mainly smaller ones though.
I think i needed to step back, calm right down, kind of give in in a way, and think ok, she wets herself, so what, I've tried everything. It doesn't bother me that much now, I just pray she will continue to get better, as the other children wil surely start to notice soon !