Pain when boyfriend ejaculates twice

We’ve been having sex for awhile, no STD’s on either (we’ve been checked). He lasts awhile, sometimes 30 mins, he ejaculates and before I can finish he gets hard and goes again. It’s rare when but when he does this he says it’s painful. He says it’s like his muscles that “push the cum out”, his rectum area, and kind of his bladder. He doesn’t have leakage, blood, red or swollen parts. Everything is normal except when he ejaculates twice. I think it’s just from using it for awhile and going twice. What do you guys think?