i have sympathy for you, OP.
I can only say, that as a teacher, your son's teacher probably sta him there as a role model, calming influence and possibly as someone who could help the child on an odd occasion if they were stuck.
I know lots of teachers who do this, myself included, although I can certainly understand why you would be getting fed up.
I've often had parents contact me, saying they don't want their child to sit beside so-and-so because of nits etc. I then discreetly move them, and move a few others as well, so that the attention is deflected.
I'd get in touch with the teacher in the first instance and find out exaclty what's going on.
(DON'T contact the HT firts, let the class teacher deal with initially, we get really ticked off when people go straight to the HT, although, sometimes, of course, it's necessary and appropriate.)
You are perfectly within your rights to request/insist that your ds sits somewhere else.
Good luck and let us know what happens.