OP - your son will still be eligible to apply for graduate schemes. Depending on what career he wants to go into, I would have thought he'd be looking at a 2017 start at the earliest anyway. In the most competitive careers, the people who start on graduate schemes the summer after graduation were usually recruited quite early in their final year of university.
That's not to say you should panic - in all of these grad schemes there will also be people who took time out to do other things or even tried other careers first - it just means that there tends to be an annual cycle for these things and in many career paths the recruitment round for this year will already have been completed.
Your son should think about the kind of jobs he is interested in and what skills and qualities they will be looking for. If he can find someone to talk to at the companies he is interested in, so much the better. He can then look at his CV and work out what he needs to do to make himself a better candidate. Often, work experience is the key. It doesn't necessarily have to be closely related to the career you want to pursue, just something which teaches you basic workplace skills. If your son can find a temporary job doing something which is relevant to the grad schemes he wants to apply for, so much the better. If not, any kind of work experience is better than nothing.
As for taking a gap year, it won't disadvantage him at all unless he decides he likes being a beach bum so much he doesn't want to come back! (In all seriousness, when I started my first graduate job I did have quite a few moments where I was sitting in a really boring meeting thinking, "God...six weeks ago I was on a beach in India..." or standing at the photocopier thinking, "Wearing proper shoes all day is kind of a pain...") He could do something "constructive" like volunteering or learning a language if he wants to, but I don't think it's essential. He will be working for a long time and I think most people interviewing him would totally understand his wanting to take a bit of time out and travel to some exotic places before setting in to the world of work. I don't think a "gap yah" will really add anything to his CV, but I don't think it will harm his prospects of getting a good job afterwards either.
The only things I would say are firstly that he should work and save and finance the whole thing himself (as someone else has said, a "gap yah" that mummy and daddy paid for will not impress anyone), and secondly, he should just be mindful of recruitment cycles and dates for applying for particular grad schemes, as if he is away travelling when all the important stuff takes place then he will have to wait another year for the opportunity to come round again.