Hi there,
You need to get your son's diabetes nurse to go into school before term starts and give all the staff who will be looking after your son a crash course in Type 1. Some of them will think they know all about it because they know someone who has Type 2. These people are even worse than the ones who think it's like an allergy! They have a steep learning curve ahead, but it is something they have to do, by law. Some schools are better than others, you may strike it lucky and have a good one right from the word go. Or you may have to work at educating them.
In the Reception class they usually have teaching assistants who (at my daughter's school anyway) take responsibility for this kind of thing. My advice is to get the diabetes nurse in, go to the meeting yourself too, and then after that cultivate a good relationship with the TA and his main teacher.
As for the hypos, as he gets older he will be able to recognise them better and tell them he is low, but until then they need to check him regularly - at times YOU decide on. My daughter is checked at morning break, before lunch and at afternoon break. And then at any other time she says she feels low (or high!). She is also checked before PE/dancing and swimming. Her school is fine about it.
Good luck with it all, anyway.
Best wishes,
Sue