Zenor77, he had the same reaction to regular yogurt that he did to any other milk product - ::zoom:: - right through the digestive system!!!
We tried a variety of products (including the lactaid pills - no luck), and by eliminating all dairy, we got his system back on track. I hope that (eventually) whatever caused this will solve itself - I'll try adding tiny bits of dairy into his diet occasionally and monitor the results. It's tough - he is NOT happy with having to give up pizza and ice cream. But, there are very good substitutions (almond milk based cheeses and lactose free ice cream) out there, and he is hardly starving!