I don't really know what to do, my 2 month old got the 24 hour flu a few weeks ago and after he got through it he had a…

I don't really know what to do, my 2 month old got the 24 hour flu a few weeks ago and after he got through it he had a hard time keeping weight on so the pediatrician told us to put him on a higher calorie diet so we've been having to make his formula thicker. Before the new diet we had him drinking 3oz bottles every 3 hours but with the new diet they recommended we give him 5oz bottles even though the formula was thicker, so we tried but 5oz was too much for him so we stuck with 4oz. He gained his weight back but with this new diet he spits up so much that he soaks 3 or 4 outfits a day. I told the pediatrician this but apparently she thinks there isn't anything wrong with how much he spits up and I asked if we could put him back on a normal diet and she said it would be best not to since he's doing so well with this higher calorie diet, I don't understand how he's doing well with it when even though he's gained his weight back, he spits up so much and it makes him miserable. So what should I do, should I keep him on the higher calorie diet or put him back on his normal diet?