Hi my ds is also hypermobile and dyspraxic - I think the main thing to hold onto is that most skills will come with time, it just takes longer. Definitely tell the school, because things like dressing and undressing for PE etc will eventually become an issue, although not really in reception as plenty of children have problems at that age.
Have you had any input from an Occupational Therapist? Our area has a parental advice clinic (in lieu of having any real paediatric OT service, but that's another story ...) so if you have a service like that you could tap into they should have experience of dealing with the local schools and can tell you and the school what to expect
Certainly I agree with Chops that elasticated waists and velcro are the way to go for now, so you're not overloading your ds with a whole load of new skills all at once. The other thing ds' OT suggested was to break tasks down into smaller chunks and start off with encouraging ds to do the last step, then work backwards getting him to do more and more steps until he was doing the whole thing.
Another thing to watch out for is that he may well get very tired very easily, so alhtough all reception children get tired and grumpy, he may suffer from that worse than the others, so be prepared for a bumpy first term and don't plan too many after school activities - he will just need to chill. Ds has low muscle tone, which often goes along with hypermobility, so has had to build up stamina gradually.
Things can look bleak at this stage, and you spend all your time concentrating on the things they can't do - however if it helps get things in perspective, my ds is now 10 - he can ride a bike, swim, play cricket, do joined-up writing, type (slowly, but it's getting there). He will never win prizes for speed or style with any of these activities but he enjoys them and can do them and there was a time when we thought he never would - good luck for your ds (and you)