The school works very hard at making everyone feel settled and happy, so I'm sure he will settle in before you know it.
There will be an Induction Day this July; an optional two days of fun science at the end of the summer holidays before the rest of the school goes back; a 3 day trip to Stratford with their form in October; a week away on an outward bound course with the whole year group some time in June I think (at which they deliberately try to mix them up to encourage new friendships); and an optional night's camping next July. By the time he's done all this I'm sure he'll have lots of new friends! Most boys know no-one on starting and several will be ex-private schools and within a few weeks none of this will seem to matter at all.
As for trips, Y7-9 have the possibility of going to France/Germany/Spain on language trips, there's a history trip to the WW1 battlefields, a visit to Alton Towers to study forces and momentum (yeah right!), optional ski-ing holidays etc.
I'm never quite sure what "geeks" means (my DS has told me it's different to "nerds", but heaven only knows what the difference is...
) Anyhow, it seems to me that all the boys at Tommies are bright, but most are just normal, well-rounded children. There seems to be space in the school and total acceptance of all types of individual, be they academic, sporty, musical, or just happy in the important business of just being a child.
Like others have said, I think you need to stress the positive to your son and don't give any sign of waivering. You must have had reasons for entering him for the exam, so tell him what they were. Of course, it's normal to feel anxious when moving schools, but STRS is very aware of this so I'm sure he will be fine.