If he's earning "a lot" he's going to be over the personal allowance, best bet is to go to an accountant and work out the best way of dealing with it in terms of self employed or ltd company.
In terms of future earnings, it's hard to say without knowing the type of content he does, is he diversifying onto other platforms not just tiktok?
Putting it all into trading would be madness, his earnings could end tomorrow and he doesn't want to throw it all on trading when he has no experience, tiktok will be partly to influence I suspect from all the day trader types. He's going to want to save some for tax, then he should do long term investment/saving type thing.
What is his post A-Levels plan, I would recommend still continuing with college or uni or a proper job, to get some kind of skills and work experience behind him for if the tiktok earnings could stopped tomorrow, he could do a video editing course.
You say you can't describe it, but like what is he actually doing? is he filming his daily life, is he filming drama videos, is it a faceless account etc? He should be aware of does he want this attached to his name in X number of years, does he want this showing up everytime someone searches him after a job interview type thing.
is the account in his actual name as a username, privacy concerns if he's showing where he lives or the area etc