Even if they move up with friends from primary, secondary school offers so much more choice of new and exciting friends that often solid primary school friendships fracture wide open and people are left out.
Usually if there is only one child coming from a certain school the secondary school visit that child at their primary and as well as the usual transition day they do in year 6 they put on at least one extra morning for those not coming from feeder schools so they know they are not the only ones and there are familiar faces from the start.
This isn't purely about your child not knowing anyone as he is making friends. Schools are usually very aware of those coming up without primary friends and there are clubs they can steer these children toward as another way to make friends rather than just in form.
It is normal for a lot of children to be anxious about secondary, it is so different to primary, lots of new teachers, moving classrooms for every lesson and honestly, much taller and full sized adult children in years 10 and 11. They have gone from being the biggest/tallest in their primary to the shortest/smallest in their secondary. My children's secondary had a separate playground for years 7 and 8 due to size difference. Plus it had its own food truck for break time, it meant they could choose that rather than queuing at lunch with the older children.
If you can get him to school on time then help him, reassure him, do whatever it takes to keep him going in. Both my children attended a school where they knew no one. They made friends in the first couple of weeks and settled. You are only 5 days in, it should get better when he gets into the routine of it and it becomes more familiar. Maybe tell him you were nervous about your secondary school, talk to him about how you felt too. Make it normal that starting school or a job is nerve racking even as adults.