Ouch, that's one hard choice. It shouldn't be an easy choice, a childhood dream of a career or a family with a many you love, but I can completely understand his side that what he wants in a relationship doesn't involve you being away for half a year. As you've said, I don't think going into the RAF and asking him to support you is feasible, it's an either/or situation.
What is the right choice depends on what happens in the future. If you stay with him, have a family and settle down, then perhaps that's the best choice. If you choose him and it lasts a few months, maybe a year or two, and then you break up, can you reapply to the RAF? Similarly if you go into the RAF and hate it, can you quit and go back to him? Possibly, but perhaps doubtful. However these can't be known, and I would even argue they're not the main issue.
The main issue is whether you can go out with him knowing you gave up on your dream for it. Will you resent him for it? Will you be able to say goodbye to the RAF and not feel pressured into it? If not, then perhaps the relationship wouldn't work. But similarly would you be able to give everything to the RAF knowing you gave up on love for it?
I don't envy your decision, there is no easy route and the consequences could change the rest of your life. I think the only course of action is to talk to him. Sod his pride that it should be an easy choice, anyone who cares about you and knows your dreams will see that it's not. Talk to your parents too, but ask them to be open and honest, not judgemental. It's not their place to approve or disapprove, but parents have a habit of offering great advice when it really comes to it. I remember when I was deciding whether to reapply to uni after missing my Oxford offer, and my parents suddenly changed from being disappointed to offering the most amazing, balanced, non-judgemental advice and supporting me through it.
Good luck!