milk supply?

I chose to formula feed my son at birth for personal reasons he is almost 2 months old now and is having alot of trouble with his formula (extremely gy and fussy with and after feedings) Doctor prescribed zantac and I am a little worried about giving him the prescription. It probably is but is it too late to start pumping or is my milk supplly gone? I feel awful about my original decision and will definitely feed future children!