My boyfriend "joked" about marriage- I'm 17?

Oh dear. You should have probably asked him if he was being serious. I would say that it's unlikely for an eighteen year to seriously propose marriage but then, considering his life, it is possible that he would want to marry you before he went away. If he seriously thinking about it, then he will probably mention it again, because he will realise that you were unsure of his seriousness. Don't feel bad for rejecting him; he'll understand that it was badly worded and speak to you about it if he feels it is something he wants to do.