As I said, he could do it pay as you go.
If he joins the air force he isn't going to get a salary which you couldn't get in civi street. For example, an avionics engineer is on 16k a year after the first year. You could get that in an average job easily, you could do your flight training part time, for example the ATPL distance learning course which you wouldn't really need to save up for because they require 3 payments over a number of months for around £700 per payment.