Wish, it sounds as though nursery weren't working very well with you if they haven't helped with this issue, and other concerns you may have had wrt your son's development. Was it the case at nursery, that he was holding on? In which case this would explain why he was flooding when he got home. How did they help him stay dry?? Did they ever get cross with him for wetting?.
I hope the school is more proactive and supportive, I would go in and have a frank and open conversation about any concerns you have about this or other issues, and ask to speak to the school senco, to help draw up a plan of action. I would also call the local school nurse office and make an appointment to see the gp, if you are concerned about other areas, ask to see a developmental paediatrician.
It is a huge amount to take in, but write down a list and take it one step at a time. Don't blame yourself or your lovely boy. You, and he, are doing the best you can.
I have a son who is disabled and in nappies for double incontinence at the age of 7, yes they do have children with nappies in juniors too! The school has gone from being supportive to being an absolute nightmare, it does sometimes feel like a constant battle.
As Miaowthecat said, it can be an incredibly depressing and isolating thing, and I wish people could just take a step back sometimes and think about how awful they are being when they make judgements on these children and their families, judgement, which ultimately does nothing to help the situation, but just adds to the stigma of an already difficult issue. My heart has a sigh of relief when I see other posters who are going through similar things (although I wouldn't wish it on anyone), in fact I might start a support thread, but not sure where I would put it where I would get posters who 'get it' and don't get posters who march through in their 'I've read an article in the Telegraph/Daily Mail/AN Other Shitrag and know all about these terrible families who pander to their children and send them to school in nappies....'.
I also work in a preschool and see children at different stages of toilet readiness, some of whom will start school in nappies. Children deserve to be treated with respect, dignity and kindness, in all aspects of their intimate care, and I hope that when I work with families going through this sort of thing, that I do so in as kind and sensitive way as possible.
Ds is going to see a grastroenterology consultant this month after years of bowel and stomach issues, I am crapping myself (which fits nicely with this thread!) about this, but hope it will be another tiny step towards resolving his problems.