having a digestion issue

my baby boy is 3 weeks and it doesn't matter what we try he is having bad gas problems and he always seems to be hungry I took him off of breast milk because I wasn't producing enough to keep up. I put him on formula the regular Similac and he seemed to be fine for a week then the next I couldn't get him to stop crying and his belly was really hard. so I then switched him to enfamil gentle ease which seemed to be a lot better for his belly and he didn't cry near as much so when we ran out I had a container of the Similac sensitive which is the same as the gentle ease after a day of being on that he is back to crying with a hard belly I don't know what else to do.