The beaches are gorgeous! Visit Herm for a lovely walk and shell-collecting. A trip to Sark is fun (you can hire a bicycle). Would your son like cannons or would loud noises be a no no? You can visit the castle (very pretty, lovely views, lots to look at in the museum) and witness the noonday salute (VERY loud).
Nice café at Vazon Bay with space to sit down. Cobo and Port Soif are nice beaches, good for digging. Port Soif has a really good kiosk with very decent food and drinks.
Victor Hugo's house is really interesting but maybe best not attempted with an autistic five year old (maybe one parent could entertain him elsewhere if the other is interested).
Please! There's 5 of us and I have a real hankering to visit Guernsey. Not sure what we're after other than relaxed and friendly people who will tolerate our autistic 5yr old, somewhere we can let him off the lead iyswim.