boyfriend cums to fast help

My boyfriend cums to fast, like I'll just be starting to enjoy it annnnd then it's over. Literally we can't go for more then 4 minutes. I've never said anything but he himself apologises every time and then he'll try and get himself going again but it never works you have to wait a bit before he can go again, but he literally just dies after cumming. He's young and in really good shape so why his stamina seems so low in bed idek. 
How do I make him last longer?