OP's boy sounds a lot like me....
I would suggest the opposite of most - if he can get good grades at A-Level, do them. Hopefully he has subjects he likes. Then, I would choose a degree based on his favourite subject (if he has one)
avoid vocational training, because he doesn't know what he wants to do, keep it as purely academic as you can, because you can do vocational traning later.
I loved Biology, so I did Biology, Chemistry and History A-Levels (History was the subject I was actually "best" at).
I then Did a BSc (Hons) in Marine Biology and a MSc in Aquaculture.
Like your boy, I never really 'knew' what I wanted to do until I did my masters so I kept my options as open as possible by doing academic things I actually liked and had a thorughly good time doing them and not really thinking about careers.
People that followed similar paths to me now are in CEFAS, The EA, do consultancy work for aquatic environmental assessments, some did PhDs, one is very senior in the "Sealife" group, there are fish farmers, some went to the tropics to farm tiger prawns etc...
I'm a sheep farmer........