Newbie - advice sought

Hi there, hope I've posted in the correct forum.

5 year old son, David, recently given autism diagnosis, not entirely a shock as we've suspected for a few years that this was the case due to behaviour.

The good: Very loving little boy, likes cuddles, adores family members, loves looking at family photos and has an awesome sense of humour - loves making us laugh! Have recently ditched his dummy at night which he has adapted to well. A few years behind with speech and understanding but getting better at communicating all the time. Can ask for things and tell us if in pain (only a recent thing) Enjoys mimicking scenes from films. Laughs a lot. Mostly happy.

The not so good: Extremely limited diet (wont use cutlery) does not eat meat, veg, fresh fruit - limited to eating brown toast, porridge (has to be spoon fed, wont use spoon himself and has to be in a certain bowl or he will push it away), rusks, mini pizza, rice pudding and fruit pouches. Not interested in "treat" things like Mcdonalds/Chips/ice creams (maybe this is a good thing, but worth mentioning!) Can be hyperactive, mostly sleeps well but can wake at random times and be full of energy! Becomes very emotional when seeing children cry - will become angry and vocal towards them and put hands over ears. Changes mood completely. Same applies to certain scenes in films where characters become sad (Disney etc) although we do monitor what he watches. Still has a warm bottle of milk at night before bed - only still allowing this as he wont drink cold milk and we want to get vitamins in him! Also hits himself when he has upset someone else (for example if he hurts his sister playing rough) or when told off. Does not comprehend "accidents" - falls over, calls the floor "stupid" and hits it, children at school brush past him and knock him accidently, same thing. Does not like loud noises, often shouts and jumps about on hearing a loud motorbike, police siren or car horn. No sense of danger, has to be watched. Will climb, run off or go near water if left alone for a second when out - finds it funny. 

So that's David for you! Complex, but very happy mostly! He is under a community paediatrician however appointments only every 6 months or so. 

There's 1 issue we'd like to put out there to see if anyone can offer any advice:

1) Toileting - David is still in nappies and no amount of toilet training (including at school) has worked thus far. Whilst he is very sensory (for example, a single hair found on his hand in the bath sends him crazy!) he has no issue walking round or playing in a soiled nappy, before one of us realises it's time to change him. What can we do?

Any help or advice would be warmly received

Thanks - Jon