He had his fun, and he's now moving on to a more grown up situation. He's signaling to you that if you want to party, it means you're in a different phase of life and that to you partying now is more important than future planning with him. It's a test, but I doubt he even knows he's doing it. That said, the worst thing you can do is try to convince him to go back into partying. You'll be wasting his time and compromising his future path for very temporary reasons, and that is just wasteful. You have to decide if you prefer the man or the party.