You will need to talk to him about it.

I know you don’t want to hear this, but I’m sure you know the cliché: he leaves, you wait for a while, then find someone else (probably unexpectedly) and are married with a kid, by the time he gets home. The fact of the matter is that there‘s a reason clichés exist: they’re VERY often what happens, which I’m sure is ultimately while you’re asking the question. That having been said, I’d like to share with you an experience from my own life. A few months before my mission, I received a blessing instructing me that I “didn’t need to bring a girlfriend on my mission.” Shortly thereafter, I wound up meeting a girl that was quite enamored with me (although I, focused on the blessing and my mission, was at first oblivious to this). When I finally gave in to her advances and agreed to date, we agreed that, because of the blessing, it would only be until my mission. The day I was set apart, we officially broke up. We’d just be friends, we said—at least until I got home, at which point we’d see where things went from there. But that’s not how things happened. For almost my entire mission, I would get letters from her at least once a week: cards, candy, cookies, popcorn, gag gifts, ties, you name it. She even wrote, one day, that she’d had a vision of our future daughter playing in the backyard. As much as I “didn’t need to bring a girlfriend on my mission,” I had brought one, and my mission suffered for it. I wasn’t as focused as I should have been, and having her in the back of my mind even impacted my level of righteousness. Anyway, you can take or leave this information. I’m not your boyfriend, so I don’t know how he will or won’t be affected by having a girlfriend at home. The one thing I *can* tell you, though, is that that level of distraction I experienced is pretty common among missionaries. Apache is right that you need to talk to him, but you definitely need to keep in mind why he’s serving a mission, in the first place. A mission is a time to be committed to Lord and absolutely nothing else, and I personally wasn’t able to do that, with a girlfriend—even an unofficial one—at home.

I'd say don't tell him you'll wait, even if you mean to. It would be more heartbreaking if you end up not being there for him later.