The university will know that parents will turn up, unless they have specifically said that parents cannot come along (very unusual - they know that parents are 'key influencers', and also know that they tend to get arsey when forcibly separated from their kids at an open day because they get worried about what the uni will say to their little darlings in their absence)
It's likely that the uni is just anticipating that he'll go along to the same sessions as you rather than having a parent-specific talk.
If he wants to go along, that's fine unless they've said he can't, but if he wants to go and see the local sights then it's absolutely fine to go solo!