What is he planning on doing in the gap year? If he is planning on leaving the country then possibly a sensible option as he will have support of school to do application & available for interview etc. Uni will usually want to know plans for year as well. The main disadvantage that I am aware of is that he might change his mind and either not want to go, or want to change the course.
If he really wants to do the S6 year I would advise waiting until S6 to apply. A number of my DDs older friends applied in S5, got the required highers and then held confirmed places and consequently had little or no motivation to work particularly hard in S6 and it was a bit of a wasted year. If the course he wants to do will accept applications for deferred places and he wants to apply now then the advantage,as has already been mentioned,is that he can then take a gap year without worrying about getting support for his application, needing to be around for interviews etc . Can't see the point if you are staying on at school. As an aside, also be aware that if your son's higher results are very good and he stays on into S6 and then applies he may well get unconditional offers anyway-this happened to a friends DD last year and she lost a lot of motivation to work!
For me, the obvious problem woud be if he changed his mind about what subject, or even what university, he wanted to go to during S6. If I'd applied to uni during S5, I would have chosen a different subject to what I eventually did - the experience of doing CSYS (old version of Advanced Higher) in two subjects completely changed my mind about which of them I wanted to carry on and study in more depth - the experience at Higher level had been quite different to how it felt to work more independently.
I don't think "knowing he has a place" is particularly necessary - he'll know what grades he has, and whether he can expect to get unconditional offers from his choice of uni and subject. And he'll know what more he has to do in S6 to increase his chances for ones he gets conditional offers at.
So I don't see any real benefit. But, tbh, if it makes him get on with it and he really wants to sort this out now, then you might be better off letting him get on with it than trying to stop him. A motivated teen is always nice to have!
Sorry - I thought this thread had died. Schoolchauffeur, that's exactly how I feel. I think he's decided to wait till S6 now becausehes'stoolazytodohispersonalstatement, so it's an irrelevant question, but thanks, all, for your input.