Boyfriend can't cum

Is there anything we can do to help this issue? He was a virgin, so I'm the only one he's been with. First time, he came 3 times in a row. He quickly was able to hold it awhile and can last a long time. Too long though. Stopped being able to cum missionary, now having a very hard time cumming with me on top and from behind. I can't even get him close by going down on him now. I've never had a guy with this problem. He couldn't even get himself off by masterbating. Maybe he's thinking too hard about it? I don't know. I told him just to relax and enjoy it, but I think he focuses too much on cumming for me. We're both getting frustrated with it and I get so tired and sore. He has no problems getting or staying hard though. He says he's close & feels right there, but nothing comes out. Help!