I Have A 4 Week Old Son That We Thought

I have a 4 week old son that we thought had silent reflux, the meds (Zantac) do not seem to work and I breastfeed. I have tried cutting out most dairy out of my diet but every time he eats I feel like I am feeding him poison! He just screams and screams afterwards in so much pain. You name it- we've tried it. We are thinking of switching to formula soon. What should I start with first? Should I go with the expensive alimentum first? Thanks!