Major dairy mystery - input please!

[Dawn and twins]

My babies have had Similac Advance since they were three weeks old. I nursed them as newborns, then stopped due to severe pain, anyway,

Dylan has been throwing up about two to five times a week since he was about 9 months old. My ped at the time thought it was due to gagging, and since he has always been a "grazer" he only takes about 4 oz at a time. I agreed with her and tried not to over feed him. She said to just watch him close and report how he is at the 12 mo checkup. I have not been too concerned since there was never a fever or diarrhea, and he has not done it for about three weeks now. Plus he is a generally happy boy.

Well, yesterday I mixed their formula with 1/4 whole milk, to get them used to the taste, and HOLY COW. He had probably 4 oz yesterday. He woke up at 4 am screaming, vomiting, and was so unhappy all morning, and kept vomiting. No fever. I called the ped and while waiting for them to call back, he did the hugest, stinkiest, very pale and runny poop ever. He was happy as a clam after that.

Then it occurred to me that I started them on yogurt and cheese at 9 mos, but stopped all dairy three weeks ago since they both have stuffy runny noses and coughs! Do you think he has a dairy allergy or sensitivity? How can this be if he has had milk based formula since birth? Is the formula just easier on his tummy?

I will stop all dairy for a week to see if there is a change, but I have to keep up the formula. Really confused. He has to have some kind of protien. He is very picky and won't eat beans, meat, eggs or tuna. All he really likes are bananas and cheerios, so I have to try to spoon feed him still and that does not go well. Any thoughts ladies?