It's not completely out of the question, he will just have to be careful where he applies. I do think he needs to be prepared for no offers though, but he should definitely try!
I looked at Cardiff and they want 9 gcse's grade 6 or above. Check the course requirements, I did dentistry so not quite medicine but how it worked with GCSEs is it was a threshold, once you meet the threshold then they will move onto the next criteria - if you don't meet it they won't look at your application. They often set it based on the applicants so say get rid of the bottom 30%of GCSE results so it can be variable. If he meets the threshold then it won't matter if he had all 9s or all 6s providing he meets the threshold.
The question is really whether he can get As at A level, with 6s at GCSE this is unlikely but not impossible. It really depends what happened, was it a handwriting issue, a not putting in the effort issue or that he genuinely wasn't capable of better. I would get the papers back so you can look at them and the marking. The problem is, is that even if he can get As at A level he's still got 5 years of pretty heavy science based exams to get through, which will be harder than a levels and he might struggle.
A levels wise 2 sciences is absolutely fine, often a separate essay subject is viewed favourably. Better do 2 sciences and an a level he can get a good grade in than 3 sciences and get aac or something.
I would think there was no point doing a gap year and then applying. Realistically he might get no offers (lots of people do irrelevant of GCSEs) and be looking at a gap year anyway and applying with results, or he might get no offers after a gap year and then have to have 2 gap years. There's no point waiting a year.
I would be focusing now on getting excellent extra curricular stuff. Lots of work experience, volunteering etc, teamwork stuff anything he can get. Have you looked at st John ambulance?