Any reason why he can't focus on the coaching and art diploma in the meantime, with a view to attending university later? The diploma might give him enough for university entry anyway, or, if he needs more time, he could go in as a mature student once he's in his twenties - he'd be likely to get more out of it then, as well, one reason why mature applicants aren't necessarily required to have particular A Levels, and the Real World experience is something that massively benefits both learners and teachers alike.
I'd never recommend a TA post to somebody as a career - they're treated really poorly, IMO. But in terms of having experience for future job applications/university, he's already got that from everything else he does/has done.
What people don't tell teenagers is that they are usually only making decisions for the next couple of years, three if they're at university. When your performance is rated upon the grades and next destination of your students, it's counterproductive to say 'meh, if your results aren't fantastic, you can do an access course or apply a couple of years later when you know you definitely still want to do it'. There is nothing wrong with him focusing on his course and sport coaching for a while. He can still apply next year if clearing isn't for him/isn't successful if he's already applied. And the year after, etc.
Nothing is set in stone at 18. He has options - and I wouldn't see TA work, part time, inherently underpaid and vulnerable as something he feels he should settle for when he has hopes/dreams that the coming couple of years could make far more achievable.