Dark green baby poop?!

My DS has been having dark green BM for about a week. He has been FF on Enfamil Gentlease for about 4 weeks now. The poop is not hard at all, but rather loose like a watery milkshake. Is this normal? I took him to the pediatrician & they didn't seem concerned because he is gaining plenty of weight. He also has a lot of trouble pooping. He strains & cries, but nothing happens. I can tell he is very uncomfortable squirming & grunting from gas too. If he doesn't have a BM in 24 hours, he starts to spit up everything at meals, probably because he is so backed up with poo. I end up having to get things going with a thermometer because he is in such pain & not eating. When he does go, it is quite a bit & then he can eat without spitting up again. We have tried massaging his tummy & bicycling his legs, but that doesn't really work. We have also tried Gripe water & gas drops, but they didn't help either. I don't know what to do. I feel so bad for him. I'm thinking of switching his formula, but I don't know what kind might help him. Has anyone else had similar problems & know what would help?