I'd go for fromage frais or yogurt. It's one of the first foods on the list of order of introduction, it tastes nice, you know how much they have eaten. Given his reaction in the past, you'll know pretty soonish whether he can tolerate it or not. I'd go for three days in a row, but I'm sure you'll call a halt if the poos begin to mount up. Ahem. If you get to two weeks without a reaction, he's probably grown out of it. I'd probably only do that one dairy food for two weeks and then introduce a different one.