Yes, specialising in tax from the start would limit future options as he won't have experience in all the other aspects of accountancy, such as accounts preparation, audit, business advisory, management accounting, etc. He risks being "trapped" in tax.
He may also not make it as all his studying/exams will be concentrated on tax so he could easily end up being bored or just incapable of the very tough, very detail heavy exams he'd have to take.
General accountancy, (ACA, ACCA) is a lot more general, so will include a much wider range of study/exams, and tax at a lower level, so it's arguable exams may be easier, but certainly the studying will be more interesting as you're doing a much broader range of subjects.
Most people, I think, would move on to tax specialism after qualifying as a general accountant if it was something that they were good at, and interested in.