Boyfriend struggles to finish


My boyfriend and I have been together for a year and a half. Everything in our relationship is pretty good, he is horny ALL the time. However, when we do have sex he struggles to finish. He has finished maybe a handful of times in the year and half, usually sex ends after 30-45 minutes from him getting frustrated and physically in pain from trying too hard. The start of sex is great for me, but after awhile it starts to feel like a chore, leading to us having very little sex, which has caused high tensions between us in the past. He refuses to see a doctor, and gets upset with me when I bring it up, he has said this has been an issue in past relationships, but has no problems when he masterbates. Any tips on the source of the issue or even how to convince him to look into it?! I just don’t want this to be an issue in the future when we are trying to conceive.