Mocks aren't harder than GCSEs as they are literally using past papers.
Saying that, those extra few weeks between mocks and the real thing gives extra time to revise, so you son still has time to bump up a grade if he revises really well.
Be careful he doesn't burn out though.
Make sure your son is doing active revision - testing himself, using flash cards, practice questions, and so on, not just sitting passively reading textbooks.
Also make sure he is targeting the areas he is weakest at, rather than just going over his strongest areas because he feels comfortable with those, but at the same time check every now and again that his strongest areas are still solidly in his mind.