Welcome to the Gloucestershire board
The general info for Gloucester schools is at the top and I am sure you have checked that out. I guess you are wondering what your DS should be getting on his practice papers?
It's not easy to say exactly what your DS needs to get for a place at Crypt or Sir Thomas Rich's. If you are already preparing and have done some VR papers, which ones are they? Some are easier than others so the kind of score you need to aim for will be different eg you should be looking at a very high % for AFN but lower for Walsh papers.
As a ball park figure I would say that you need to be getting at leat 75% + minimum and 85% + to be more confident on, say, the GL assessment papers. Obviously you could be still building up to that for Oct. (Other people please chip in if you think I am not right on those %)
It might not still be the case as Crypt has been gaining an excellent reputation but historically it has been harder to get a place at Tommies (STRS) than Crypt so this would mean you would need a higher score to pass for STRS.
Anything can happen on the day so scores on practice papers may not be a prediction. Definitely aim for enough speed to finish the papers, look at exam day tips (there have been good threads on Gloucester board so look back to around the last exam day also Patricia has good tips for speed).
Apparently there is a mock exam for Gloucester on 11th August (private company though, not run by the schools) I have never used this so can't say if it is worth it.