Moving to middle school?

[Secondme]

I am not really sure where to put this. My children do first, middle, high school and this is about moving to middle school. I suppose it is like moving to secondary school, although the children are still primary school age.
Anyway, ds2 is about to start middle school. He is in year 4 now and very quiet and has barely any friends. (He doesn't mind this, he just prefers being alone.) ds1 and dd1 were very confident when they started middle school and had loads of friends going with them, so I wasn't worried about them.
But I am quite worried about sending ds2 on a bus on his own (well with siblings but they won't want to spend loads of time with him). Any advice from parents who have been through this on how i can prepare ds2 for it and how I can stop myself constantly asking him about school life? He gets annoyed.

My DC do First, Middle and Upper Secondary.

The Middle school is yr 5 to yr 8.

Yrs 6,7 and 8 are arranged just like a secondary school. Ie subject teachers and tutor groups.

The beginning of yr5 is arranged more like primary for the first term ie they have their tutor for most of their subjects and they stay in the same classroom. Any subject teachers that they do have come and get the class and take them to the relevant classroom iyswim.

After Christmas they start moving classrooms and having subject teachers.

They also go by school bus to school. 75% of the children go by bus so for the first couple of days a teacher is in charge of making sure the yr5's get on the right bus.