I've no idea how the RAF functions nowadays; my wife was in around 1966 and she was treated utterly abysmally but, hopefully, things are better. One of her room-mates married an airman and about a month later he was sent on an unaccompanied posting to Gan. The marriage collapsed...
I think you have to be a special type of person to accept military life and I'm sure a lot of people enjoy it immensely; it wasn't for me though. I spent a while with RAF ATC in Malta and there were some great characters... but I also recall some jumped-up, spotty little pilot officers doing their level best to make fools of much older NCOs who were ten times better at the job!
Regarding career progression.... is that what he wants or what YOU want? Maybe he would be happier just talking to aircraft all his life, as I did? Opportunities to progress in civil ATC are fairly plentiful and the top salaries are very good indeed considering one only works about half the year. Top salary for a working ATCO in NATS at some units is around £80K. Management types can get £100K and above.
Good luck and I do so hope that you both make the right decision...