Boyfriend finished before me, and then feels bad. How do i encourage him?

My boyfriend and I have been dating almost 5 months, and have been having sex for 2. A problem we’ve run into is he comes before me, and has trouble getting me there sometimes Its never because im uninterested, I always am and I love having sex with him, but it just hasnt happened yet. I was his first, but he wasnt mine (which doesnt bother either of us at all). Sometimes he gets discouraged when it happens, there have been times where we’ve gone for over an hour (but i still dont come because we get interrupted sometimes considering we’re teenagers and our parents come upstairs or something so we have to stop and not get caught) and other times he comes within 15 minutes. He does go down on me and it helps, but he always feels discouraged if he comes sooner than I. He’ll say “you deserve better” and that he wants to help get me there. What can I say to him that encourages him that i AM enjoying it, and that its just something we’ll have to practice and work on? I try to explain that to him but I know he still feels bad.