Don't worry over much about the effect of tutoring. If you had been "training" him for Grammar from Reception then you may have cause for concern that he won't cope. As it is, you do need to prepare and you would be more upset if your son didn't obtain a place owing to lack of tutoring and you then see children who have been allocated places achieving lower SATS results
With regard to time, have you tried sitting with your son when he does a paper and observe his technique. Time him on each question type so you can see his strengths and weaknesses. I was surprised when I did this with my daughter. You can then work more specifically on the ones that cause him problems. Is your paper structured so that you have say 100 q's in 45 mins or are the children presented with sections individually timed.
If it is not individual time sections you will be able to advise on which questions to skip over and go back to - with our daughter some questions were taking her a long time even thougth she was getting them right. By using this technique she skipped the penultimate section and was able to answer the last 10 questions, and went back to the question types we had identified that she found difficult. By the end of the paper she only left out 6 out of 100, also very few finished the VR paper this year (if any) so missing a few didn't make too much difference.
Hope this helps