Baby constipation

My son is 2weeks & 5 days old (born 3 weeks early). I am breast feeding.. and since Thursday, he has been having A LOT of trouble pooping/passing gas. He has been up all night crying in pain trying to push stool out and pass gas. It’s hard seeing him in so much pain😩 What can I eat to help relieve his constipation? He was pooping every two hours and then just stopped on Thursday for 15 hours. Now he only gets about the size of a quarter out every once in awhile as he cries in pain. We exercise his legs frequently.. Any advice will help so much😭