Missedcall974:
I think you need to step back and work out what being in that form actually means. Each school is different some forms are set such that all classes are with those same people and others are just set for that brief session first thing in the morning (attendance/ information/ PSHE) and then you are off to classes with a mix of kids from all the different forms.
So my advice is work out whether his friends are in fact in some of his classes. If the problem is that he isn't seeing any of his friends - then you need to really ask yourself if that is a good thing. Is he easily distracted if friends are in his class? So, in a class without his friends he has to pay attention. Is he ahead or behind his friends in terms of academic achievement - so he's in a form which can meet his needs as a student?
If any of this applies - try to persuade him (and that might be a big ask) that he has the best of both worlds - he can see old frirends, but also can make new friends.
If however, he's in a form with the kid that bullied him all through primary - you are absolutely within your rights to raise this with the school (as the primary also should have pointed this out).
Finally - on another discussion about 'making friends in Y7' - people have been suggesting (including me) that joining clubs really helps with meeting people & making friends through shared experience. clubs should be starting up and most schools in Y7 run clubs during lunch, possibly right after school.
Encourage your DS to look into joining a few things - it's good for pursuing interests, can make you stand out from the crowd on 6th form/ uni/ job applications & is character building (e.g. DofE awards).
HTH