Not sure that IQ itself is much help - except that any presumably anything you need to teach them will be picked more quickly!
Best bet would be to get hold of past papers from CSSE & see how he gets on with them without any coaching. You can see from other threads what marks other boys were scoring on the papers to get a place st CRGS. Remember that it does vary from year to year & apparently children drop average of 10% points in the 'real thing'.
If he gets over 90% then would suggest you dont need to do too much. English - need to make sure has grasp of basic punctuation - especially speech & needs a wide 'traditoinal' vocab (probably covered at a good prep anyway)
Maths - would imagine will be fine. Just check OK with problem solving as some preps aren't so strong on this.
VR - very much a matter of technique. Some children take to it naturally but if he is bright shouldn't take too much to explain the techniques needed for different types of questions that he doesn't 'get' immediately.
Really important that he can use multiple choice answer paper for VR & has technique to answer as many questions as possible.
Weighting system means VR twice as important as maths or english.
Hope this helps a bit