I know we're not supposed to say bossy but they bloody are.
DS has been unhappy on his table all term, 2 boys and 4 girls. One of the girls in particular is very bossy, always pointing out where/how/why DS is wrong or needs correcting. Eg you're not holding your pen correctly, you haven't done enough work, do it this way not that way. Teacher is aware and is aware of um controlling personalities on the table.
It's all came to a head today, DS in tears because the main girl has been on his case all day. Teacher is off sick and there's a sub in the class who has told him to just 'ignore it'.
I've armed DS with saying firmly "you are not the teacher leave me alone" but she just mimicked it back to him
I think that you need to speak to the teacher again, pointing out that what is going on is bullying, and that it needs to be dealt with. The girl in question is probably not aware that she is bullying, but the adults in charge need to make it clear that what she is doing is causing a great deal of distress to your son. What does the schools anti-bullying policy say?
I don't think there is much you can do until his usual teacher is back. Then have a word as soon as you can, and pout. Out that this has been going on for a while and was far worse with supply teacher, and ask what can be done to fix things (thoroughly, so there isn't a recurrence any time the class is with a different teacher).