I think educationally it is a great thing to study a wider range subjects and the course is great preparation for university, maybe even better than A-levels - all the research projects and critical analysis. But it is a lot of work... My eldest says the first few years of med school were easier than doing the IB (although to be fair, he did have all the coursework due plus the UCAT/BMAT and interviews going on at the same time) 
What does your DS think he might want to study? I would look at the IB and A-level offers at the universities he might be interested in. Some universities make lower offers for IB, and it is worth a lot of UCAS points (but not very many universities use UCAS points) but, often, getting the IB offer is harder than the equivalent A-level offer eg Cambridge offers are usually 42/45, almost impossible if you have 1 weak subject and there is always the risk of moderation pulling marks down on coursework. If he needs very high marks (A*/A) for the course he wants to study, he will need to be an all rounder to get the equivalent offer in the IB. On the other hand, some US universities accept it as credit for an entire year of university.
My DS had offers of 36, 38 and 39 with a minimum of 6 in all HL subjects from courses that wanted identical A-level grades. He rejected an offer that wanted a minimum of 5 in all the SL subjects too - it seems ridiculous that a university would reject a science student with 3 x 7 in HL sciences because they got a 4 in French but accept a student with the same overall score but only 3 x 6 at HL. To translate that, it's the equivalent of taking a student with 3 As and an A in GCSE French but rejecting a student with 3 A* just because they got a C in GCSE French 
If your DS is more of a last minute reviser who can pull off good grades, the constant slog of assessed coursework might not suit him. On the other hand, if you do put the work in, good coursework marks take the pressure off the exams.
If he isn't a natural linguist, we were advised that it isn't a good idea to take a language they have already studied because the standard is quite high. It's easier to get a good mark learning a new European language ab initio. Could your DS manage to get to GCSE level in 18 months? What languages are on offer?
I would also ask about specific subjects once your DS knows what he would chose eg I believe Higher Level Further Maths is considered very difficult to get a top mark.