Both my sons went to a secondary none of their friends did.
If this is a state school there should be a day in July where all the year 6 children go up to their chosen secondary and spend a day in their new form with their new class mates.
My son also attended an extra morning with children coming from non-feeder schools, so some team building games and more familiar faces.
If you can at any point, without looking like a stalker, park up nearish to the school to see the kind of rucksacks, coats and shoes the boys are wearing I would do that.
Mine have never had expensive backpacks, more like a £25 laptop bag off Amazon 
I will tell you that both of my sons, introvert and quiet Ds1 and quirky, nerdy Ds2 both made amazing friends.
But breathe, he will be okay. He will not be the only child who doens't know anyone. He can walk to lunch with whoever he sat with in the previous class. There should be a school trip fairly soon after they all start which is designed to break up some primary groups and introduce new kids to them.
There are loads of lunch time or after school clubs he can join to make other friends too.