I had same issues with DS1. In end I left him and at 3 years 7 months something clicked and he wore undies and never looked back! We only had 2 wee accidents and both time because excited to see someone and forgot on way to toilet and a week of poo issues as he was scared to do it toilet.

On his 4th birthday he told me he didnt need a nappy at night now he was a big boy and have had NO accidents.