First Times

My boyfriend and I are both pretty young (he’s 19 and I’m 18) and we just started having sex. The first couple of times we did it he was able to stay hard through the whole thing and finish. But the last 3-4 times, after about ten minutes, he goes soft and he can’t get hard again. Neither of us are sure of what to do or what’s making him go soft. He says he into it and he wants to do it and he’s just confused. If anybody can give me some advice or just let me know anything that’d be a big help