There are a few careers which don't require GCSE qualifications, nevertheless, those jobs are more likely to go to those with qualifications even if they aren't required.
Oh, and you'd think things like dancing don't require any qualifications but I know a few people who went to stage schools to become dancers, and they all needed A-levels. Obviously there are a handful of places which don't require any qualifications at all.
At the end of the day it's his loss. He's more likely to succeed in whatever he plans to do, if he gets some GCSE's and then goes to college or whatever. I'd say his chances of getting a job without any qualifications are pretty slim, considering how hard it is for qualified people to get jobs.