Can I be radical
and suggest that he doesn't necessarily need to go to university at all, at this time in his life, if he doesn't know what he wants to do there?
Or he could take a gap year after A Levels, and work or volunteer to get some experience, and to start thinking about what he might want to do. He'll also be applying to courses with qualifications already in hand.
Or he could do some thinking about what he enjoys doing, and what he's good at. Both in terms of hobbies/non-school activities, and school studies. Where there's an overlap might be where to look at degree courses.
But really, if he doesn't know what he wants to study, and he doesn't know what he wants to do after university, university at this point in his life may not be a good thing for hi,
And it's definitely not a good thing for those who have to deal with him at university, frankly.
You ask about open days -- take him to as many as you both can stand! We love it when people come to find out about courses and facilities. But we can't answer the question "What should I do at university?" If he's not sure, he shouldn't be going until he is. And there are lots of ways of helping him work out what he might want to study.
The other thing is, I don't necessarily think it's so important to have a definite idea of what he might want to do after university. I think that some of the jobs our DC may do in 4 or 5 years time haven't even been invented yet. And it's likely that most professionals will do several different kinds of jobs over their working lives.
So my emphasis would be on finding out what he enjoys doing, and what he enjoys studying, and seeing where they overlap.