he's normal. children develop differently, and the ones with older siblings do learn quicker. girls also tend to be quicker. some want to be independent, some like it done for them. also chubbier children find it harder as they simply can't reach their bottom! bless them...
it's just that there's a hands-off policy in most places as a result of child abuse/health & safety over-reaction..
At my sons playgroup kids start at 2 and staff are quite relaxed about toilet issues, (and of course change nappies and help the kids with wiping their bum or clean them if they soil themselves), but will require you to put sun cream on before start of session as the carers are not allowed/supposed to do so.
why can't they help? It's not practical, there's no time - that I'd accept, but hiding behind a blanket policy is just insulting my intelligence!
So I put it on at 8.15, but surely it rubs off by 11 am and needs re-applying?! how can you expect a 2-3-4 year old to do that safely? ridiculous! I still do it for all our kids, oldest is 11. they maybe capable, but I don't want them to rub it in their eyes.
she didn't frown, but made you feel uncomfortable, what a cow! does she have kids? take no crap from her, your boy is fine, and will catch up soon, no doubt! anyway he might prefer to do it himself than a stranger doing it for him...
best thing is to try to catch him up a bit. make him wipe his bum, he might get messy, but he'll learn to get better. make him put your coat on you to practice. or your socks. make it into a game, no dramas. you are doing a fine job, don't let anyone tell you otherwise.. Also I bet he's ahead in other things or he's better behaved, so don't worry!
oh and a friend was still dressing her five year old for school, and people were curious why. turned out she was so used to dressing him, it never even occurred to her to ask him to do it for himself... the next day she did ask. he managed it, of course!