Hi oldmegan,
I can't help you with the repellant, maybe googling will bring something up, but I was just wondering, if its just him thats being pestered by them, could the Wasps be attracted by something he's wearing, (they have a good sense of smell), a deoderant, the type of soap etc.
Because of the type head hair I have, I have to use only a cetain type of lotion, and that attracts Wasps, I've learnt not to bother them, and they don't sting, but obviously your grandson is too young for that, so I can understand your worry.
I wish you well in your search.