As a careers adviser,
A) I'd be wary of assuming what teenager tells his parents or teachers about his guidance interview is 'all' that happened - quite a lot of exploration might have occured before suggesting he look at the info to get ideas of what he could do with his strengths and interests, after they'd identified he was struggling to make decisions because of having long term goals. Or of course,it may not have been a very good careers interview! But it's not realistic to think an unfocussed young person is going to come out knowing what they want to do as a career after 40 minutes.
B) My advice would be not to panic about not having a long term goal at this stage. 3 main things to consider: what is he good at, what he most likely to be really interested in studying and is he ruling any potential careers out? The first 2 because the jump to A levels is a big one and he'll need some interest to keep him motivated.
In terms of careers, as others have said if he has any leaning at all towards maths and physics, and assuming he can get at least good B grades at gcse or whatever the equivalent in England is, it would be a shame to drop them and rule out scientific courses at uni. There's not much point taking physics without maths though, as most engineering and physics degrees will ask for maths.
However, if he also has an interest on more humanities based subjects, definitely include them too for breadth of skills and variety for him. I don't necessarily subscribe to the idea history is automatically more useful than law, psychology etc. He's learning useful transferable skills either way, so he could go with whichever subject matter interests him most, but he does need to really research the course content for anything he hasn't done at gcse to make sure he doesn't have a false idea of what they will be like.
Of course, if he really can't find 3 subjects he is interested in, he should also be asking does he definitely want A-levels? Are there any vocational courses or apprenticeships that inspire him more? I suspect not as he is quite unfocussed and these would be narrower, but always worth the question!