Too young and he will be gone for a while. Wait till he comes back before making any commitment or accepting one. He may have changed too after the deployment substantially. Can start life at 28 years knowing where he stands then. He does not know what he plans to do now in five years nor can you expect him to given it is a life changing experience.
Just say to him what you typed. A good relationship is ba<x>sed on honesty and communication. Hey he might feel the same as you !!
Living with someone for only four months, I personally think, is not long enough to determine if they are a suitable mate to be having children with. That is a huge, huge thing to be talking about. And three years is not long either. But I would DEFINITELY express your demands to make sure they are compatible with your future wants and needs. And you are still only 23yrs old. in 3yrs you will be different then you are now and you will want differently to some extent for sure.
Be honest about how you feel about the situation. Though honestly...you can have kids whenever you like in life no real rush.