Tell him to have fun and send postcards. Then find other guys to date.
What? You’re supposed to wait for him? You certainly can, but why should you? Did he ask you to join him on his adventure? No? Then he made the choice to travel without you, which is fine, but it doesn’t mean you should sit at home with a candle in your window as you await his triumphant return.
He’s seeing the world, expanding his horizons, meeting new people, growing and learning and living. More power to him. But you should be able to do the same at home in his absence: go out, do fun things, meet new people. I know it’s not quite as thrilling as backpacking in the Andes, but it’s better than banishing yourself to the tower and polishing the lock on your chastity belt every night. You deserve to grow and expand your life, too, and part of that is dating other guys if you want.
I don’t know if he expects you to wait for him, or if you are pressuring yourself to do so. You can wait if you want, but who wants to live in limbo? Not me. Life goes on, with or without our participation. Me, I want to be on that train when it leaves the station.
Look, if you really like the guy and want him back when he returns, great. You don’t have to break up; you can call it a hiatus, and he can look you up when he returns. But I think it’s unreasonable of anyone — him, you, me, Betty White, Bigfoot, anyone — to expect you not to date around in his absence.
So decide what you want to do, and do it. You don’t need his permission. He made his choice, and now you have to choose what’s best for you.