5 Weeks Old Is Constipated. Not Cured By Milk Of Magnesia. What Should Be Done?

I just want to know what else I can do for my 5 week old baby for constipation? My pedi told me to give him 1/2 teaspoon twice a day of milk of magnesia but this has been going on since he came home. He is still super gassy and doesn t poop on his own without it and STILL has to be stimulated at least twice a week. We even changed his food to sensitive similac and it doesn t seem to be that different. Thank you,