I changed from a non-selective mixed private school (yeah I know) to a massive co-ed state 6th form college, as did my sister.
I really benefitted. As someone already mentioned, it really prepared me for studying at Cambridge where I had one hour of compulsory contact time a week and thus had to manage my time and be very self-motivated about ploughing through my reading list/ getting my essay done/ attending the relevant lectures even though not mandatory.
At 6th form we only had to turn up for lessons (no school day as such), and if you didn't hand in coursework/homework it wasn't chased. You just got zero. The classes were still reasonably small (15ish) and i really enjoyed it.
However, my sister did the same, didnt cope with the unstructured environment and massively underperformed in her A-levels
I wouldn't worry about the grades. If anything, the state sixth form will probably have opted for the less rigorous boards so he might do better. Also, and completely cynically, he'll be a state school applicant for the purposes of Uni stats (look at port of last call) so that might actually help him.