If you have prepared your son for the Bucks tests, he is well prepared for the VR in Slough.
I am not sure about the 21 questions types. I have assumed for a long time that there was no extra tyoes in Slough, but in reality, I don't kmow.
For maths, if you have finished the NFER papers, you can try the IPS 35mns or 6mns papers. They are very simillar to NFER, just a bit harder I think, so don't get disheartened if your son scores less with NFER.
For NVR, yes you should use the NFER tests. For more advice, read this thread:
There is a lot in it. I think that all the comments apply to NFER type tests.
Most of the recent posts in the Verbal reasoning section would be relevant for you I think.
Please ask if you have any more question.
Best of luck