Dinosaur Theme ideas
With children of that age, I don't expect many of them will be happy sitting still for long - so good that you are considering doing things like pass the parcel in small groups.
Can you find out from his mum with regard to what he can and can't eat. You should actually be asking everyone attending the "can't eat" question.
I am reading Tony Attwood's book on Asperger's Syndrome, and he says that HFA is the same thing. So may be worth looking at information on Asperger's Syndrome to get a better idea of the boys situation.
Some children love Water... so as you already have some garden things, consider having a water tray - perhaps with floating dinosaurs.
Could even try a game of fish the dinosaurs out of the water tray, using a net, or something else. How many dinosaurs can you get out in 30 seconds?