What's going on with my 2 month old?

My son will be 2 months old on December 20. He was born perfectly healthy and normal. I started out feeding and supplementing with formula. But by the time he was about 2 weeks old I had quit feeding. At around 3 weeks old he began a strange behavior: grunting and straining like he was constipated. But he's not at all constipated. His poop is normal in color and consistencey. He has a BM about every 24-36 hours, which is great for a formula fed baby. But he continues to turn red in the face, grunts all the time, strains, and cries like he's in pain. He will be totally asleep and just wake up screaming. It's so strange. Also he has alot of gas and gets hiccups at least once a day. He eats alot too, like 5-8 ounces at a time every 2 hours. He never seems full. I know that I'm making the formula correctly, This could be my fault because everytime he cries I give him a bottle, but that's the only way he is quiet. When he is awake he is crying unless he's eating. He refuses a pacifier. I don't know what to do. Please help!!