Would really appreciate some advice as I'm stumped. Will try to summarise briefly - quietly introduced a potty and books about using it 2y6m, he started to show an interest and do occasional wees about 2y9m. We did some pants off time, slowly getting more wees on the potty or toilet and started full time in pants about 3y. He fairly quickly got the hang of doing a wee, less quick with poos but now has good control and can recognise he needs to go, deal with clothes etc without any support - the main issue was always leaving it to the last minute because he didn't want to interrupt his play.
But then he gradually started to refuse to go, without any trigger that I can think of. We introduced a reward chart (10 stickers for a video) and stopped reminding him as this seemed to be counterproductive. If he has an accident we take him to the toilet, clean up and change clothes, and calmly say 'next time please try to do it in the potty'. But he's now 3y6m and getting worse and worse - today not a single wee in the potty, and he's refusing to go even when he has started to leak. I know it's important to stay calm and relaxed but it's so frustrating knowing that he can do it. Any advice?