Poop is inconsistent

My son is 3 weeks and a day old. His bowel movements are so inconsistent. When he first came home, he pooped 7 times in a day then it gradually lessened. Now he'll poop once every other day to every two days. When he poops, it's soft but he cries like he is in horrible pain. Looking for some suggestions and remedies. I tried the tip of a thermometer with Vaseline this morning. Nothing yet. Thanks in advance.