5week old has belly problems!!!!

He has gas constantly and cries with his belly. Tried changing milk but he was born prematurely so it was originally on Neosure Milk for premie babies but I changed it 3/4 times so see if anything else would help so I finally changed it back to neosure because again like I said he was born early and doctors want him on that til he's 4months but I've took him to the doctor and they do nothing! He's constapatied and has gas. What can I do?