He might find a gap year more appealing if he does something a bit more exciting with it. Working abroad for example or volunteering or just travelling.
However I think if he wants to go to university you should support him. Not knowing what he wants to do at this stage is hardly unusual, in fact often it is preferable to have a flexible approach rather than a fixed plan.
He's clearly bright if he's getting Bs without much effort so a degree is unlikely to be a waste of time.
If he enjoys Spanish then I would encourage him to look at joint honours and doing Spanish with another subject if he doesn't fancy a language degree.
Language degrees are highly thought of and sought after by employers so don't discount that. It would lead to lots of employment opportunities, not just translator or teacher.
Humanities aren't the dead end you seem to think either, and he doesn't have to do something he has an A-level in, he could do a subject he has an A-level in or something connected. So say he has history A-level he could consider, a history degree or something like Ancient History, Archeology, Spanish and History, Ancient and Medieval History, Egyptology and History, History and Politics, Modern History, History and American Studies, History and International Relations etc.
As an example very few law undergraduates have an A-level in law, so he could consider a law degree but a fair percentage of solicitors and barristers have degrees in other subjects with humanities, English and Languages all being well represented. Subjects like politics, business, international affairs, might be worth considering.
It is entirely legitimate for him to pick a useful subject that he is good at and decide what to do with it later, he will still benefit from the degree and improve his job prospects. I wish I had done that!!!!