My almost 3 month old is spitting up like crazy!?

He's been on enfamil newborn, enfamil gentle ease, enfamil ar, and now they are trying him on nutramagin. I put 1 tsp of rice in his bottle and he didn't spit up for like 3 days, but then it started again. Now my pedi wants me to put 1 tblsp in each bottle. He was on Zantac and now Prevacid. He is for the most part a happy spitter, but he does straighten up and arch in pain after most feedings. But not near as bad with the meds. They don't want to do an upper gi yet. The reason it's a concern is b/c he's not gaining weight like he should. He spits up all that he eats! He was born at 7.13 and he's just now 10.10. He'll be 3 months on the 4th. They say he's in the 5-10% range and they'd like him to gain more. So if this doesn't work what will they do? Anyone have similar problems? What did they do?