Do you mean, it's not worth the exam fee/ a bit of tuition because it may not pay back in a job directly?
That's the wrong way to look at it, if I can be a bit direct with you. Your DS is about to enter into two years of 6th form and then probably 3-4 years of a degree plus perhaps grad study. It's hard work and they need to feel respected and valued while they do it. If you interfere in his subject choices, it needs to be for an excellent reason.
It would one thing to say, none of your A level is a strong RG subject; you mist have at least two. But he has three already plus psychology, so that's nor an arghument.
It would be another thing to say, I am concerned that it will dilute your effort and put your other grades at risk. but you say that's not an issue.
So you are just left with, meh, is it worth £50 entry fee, will it get you a job? I think he would be well within his rights to resent that as a reason for exerting your authority and preventing him doing something he wants to do.
As for how useful it will be: not specially. If it's ever relevant that he speaks fluent Hebrew, I doubt they will care if he has an A level in it: being bilingual will be the main thing. Having two MFLs isn't wrong.
Let him do it if he wants to. Save your authority for when it matters. Show him you are proud of him for wanting to do well.