Really feel for you and your DS, OP.
My DS1 had an extremely similar 'BF' relationship at the same age. It started with the blank days, the nasty criticism, but when they got to high school it moved on to hard shoving, tripping up, jabbing, etc along with nasty personal appearance based bullying. Other kids started joining in too and it became ridiculous, especially out of school grounds and at the bus stops where the school were limited in their contol. Fortunately, we managed to move DS1 to another school and he could leave all that behind him and make proper friends and regain his confidence.
Like you, though, we live in a small community and it was an extremely difficult time for everyone once DS1 got brave enough to talk to his teacher about what was going on. The bullying got worse until Ds1 moved school so he was too scared to report again, and yet the other boy changed the story amongst their primary school peers, who live around us, saying that Ds1 had been the bully! DS1 has been fab though and maintained a dignified silence about it, only denying untruths but not talking directly about the other boy's bad choices and actions. I had to maintain a professional relationship with the child's mother where prviously we had been good friends and that was really tough.
The temptation to go in all guns blazing was hard to resist (and in fact it was very difficult to stay quiet until ds1 was ready to talk at school) but we had to keep the balance of protecting DS1 and maintaining a peaceful co-existence in a small village for decades to come. There was cctv footage of the bullying at the bus stop, and another child had filmed the shoving etc at another bus stop on their mobile but I didn't pursue the cctv and refused the mobile footage because by then DS1 had been offered a place at another school - I am glad we made that decision as things worked out well for my DS1 and ultimately it was not my job to correct that child's behaviour or help him with his problems.
OP, I would use some white lies to get out of your short term plans with this child. I would have a discrete word with the teacher about what is going on, asking for the sake of protecting your ds that they only observe and step in as necessary but not call up the kids based on what you say. Chances are they should be aware of some of what it going on. Are the children all going up to the same high school? In a bigger school it will become easier and more natural to make new friends and move on. However the older he gets the more your ds will be alone and need to look after himself so now is a good time to encourage distance from bad people. Encourage your ds to also try other clubs that do not involve these 'friends' so he can broaden his socializing. I know that the gentle and considered approach is not always popular on MN but there are many aspects to life that need to be looked after: the wellbeing of all the children, your family life, your community life, etc and a peaceful last few months in primary.