I'm 16, he's 21 But he doesn't know how old I am yet.?

I'm a senior in high school and graduating in 2 weeks, yet I'm only 16 for another 5 months. I've been Getting to know this guy as a friend for now and have known him for about a month, however age hasnt come up yet so I don't think he knows I'm 16.. He probably figures I'm 17 or 18. We Both have super high morals standards and we won't be sleeping together...seriously. But I plan on telling him my age when he gets back from a missions trip. What do you think his reaction will be...? I mean of course I'm going to tell him my age now before things get serious at all. It would be immature not to... ehhhh, thoughts on the whole situation?