What can DS do to cope with pressure

[Threeschools]

DS1, in y8 in an independent selective boys school, is feeling the pressure and getting anxious. I thought that since he was offered a place at that school it was because he was able to cope with the fast pace of lessons, but now I wonder if he is going to burn out? The boys are expected to do very well in external examinations and I have just found out that about 50% have extra tuition in maths. He has made it to the A team in the school major sport and is now frightened of letting his team down and thinks it is probably more fun in the B team. My point is if he is already freaking out, how will he cope with the GCSEs etc? Is it normal to feel like that or should I take him out of there and put him in a mixed ability school? Our local comprehensive requires improvement so I am not sure he will be better there. At least at his current school he has good friends and everybody is very nice he says. He was disappointed because he got average in his end of year 7 exam, but average in that school where more than 90% get B and above is fine, he needs reassurance and to believe in himself a bit more. He finds maths hard, he is in the top set. Should I ask for him to be moved down a set or give him a tutor? This is getting ridiculous frankly, because he is very able and shouldn't feel like that. It is hard to see him like that.