The first thing he would have to do would be have a medical examination with an AME and then submit the medical certificate to the Ministry of Transport and Civil Aviation for the issue of one of these:-
I can't remember how much it cost (I'm sure it wasn't free) but armed with this he can now be sent solo by his instructor when deemed ready.
Can't speak for an ex-military pilot but when I got my PPL in the sixties the syllabus required a minimum of forty hours flying (thirty-five if you were training at an approved flying school) of which a minimum of ten had to be solo. In those days one was also required to carry out a solo spinning exercise and of course all the other requirements of the syllabus had to be met. The training culminating in a Final Handling Test with the CFI (although I think we called it a GFT in those days).
Hope this helps.