Belly Troubles

My little one is 8 weeks tomorrow, he is formula fed-taking about 5oz every 3-4 hours. He seems to be having a hard time having bowel movements. He will cry out with a little pain and then strain. When he does finally go, it isn’t hard like he’s constipated so I’m not sure what could be causing him so much trouble. Any ideas or tips on how to help? We did switch his formula to the sensitive kind and that seems to help with the gas but not the other.