Not eating enough

I need help. Lately my 7 week old (as of tomorrow) hasn't been breastfeeding very well only wanting to do 5 - 7 mins each side, I try to give him formula after and he still will only eat 2 oz and he should be eating between 4 -5 oz from what I'm reading, which he had been doing a few weeks ago. I know hes not getting much from my breasts because I have always had low supply so we have always had to supplement and everytime I pump I get between 1 and 2 oz each side. Hes also been spitting up more then normal this week, and hes been on zantac for 3 weeks now. Just not sure what to do I'm afraid hes not gaining weight. Any suggestions or thoughts..