What Can Cause Frequent Hunger With Constipation In New Born?

My son is 4 weeks old and is waking up every hour- hour and a half demanding to eat. I have tried everything else (changing, holding, playing, rocking) but the only thing that settles him is a bottle. Each time he eats, he finishes 3-4 ounces. I am worried about over feeding him, but I also don t want to leave him cry when he is hungry.Also, he is constantly straining and grunting as if in pain or constipated (even in his sleep). He is having regular bowel movements (3-5 small bm s every day). They are a little thicker than mayonnaise in consistency and were mustard colored but are now a greenish in color. It sometimes looks as though there is some mucus in his stool. I have tried holding his knees to his belly and bicycling his legs; I have also tried Gripe Water; nothing seems to help. I don t know what to do, my baby seems so uncomfortable.