Finally ds had an invitation to a birthday party. He had a fabulous time, politely refused all food, thanked the host for having him, made some friends (not quite 3, so the first time I've seen it happen).
When the cake got cut, all the kids got to grab the sweeties off it, but ds got a slice of bread instead and was delighted.
And unrelated to allergies, but also super - he went on someone else's toilet!
See it's not hard to invite an allergic child.