True, but speaking as a bone idle ex-comp boy I would say that it will be much more likely that he'd maintain that initial enthusiasm through to GCSEs and A-levels at GS than at a comp where he can, if he wishes, subside quietly into the mass or (perhaps worse) cruise along making 70% effort, stay comfortably ahead of the pack and then struggle with the demands of A-level. Not that I'm speaking from 30-year old experience or anything you understand, I wouldn't behave like that
If you think that a comp these days would let a bright boy rest on his laurels and cruise along making 70% effort Mike, you are so wrong. Comprehensives live and die by their league tables, and they track all pupils from the moment they get in there. The trouble is, you are speaking from 30 year old experience, when it was quite likely you could get away with minimal effort, just like I did too.
The OP is from Gloucestershire, where the superselective system put just 5% of children into GS, and some travel from well outside the county too. This means that there are many excellent comprehensive schools. It is sad if the impression created by this forum is that any child not in a GS is doomed.