3 month old infant not drinking as he should?

He's always been fed expressed milk and formula (not producing enough milk - medical reasons). He has a milk allergy and is on enfamil nutramigen. Has also tried similac, similac soy, and simiac sensitive. Basically he drinks about 2 oz milk and 2 oz formula (since 6 weeks old approximately the same amount). I have tried to give more and he refuses. Recently, he arches his back, stretches out his legs and screams as if in pain. I have to force him to eat the amount that he does, either while sleeping or by talking softly. I have to work very hard to get him to drink for 3 out of the 6 feedings. He sleeps about 9 hours a night and eats about every 3-4 hours. At birth and at 2 month check up he was at 50% for weight and length (height). Any suggestions?