Unfortunately this happened to me in school back in the early noughties. The staff knew I had epilepsy and one of the lunchtime assistants told her daughter, who then told other pupils, who then approached me to ask if it was true.
I would certainly be putting in a complaint to the headteacher and if I didn’t get a decent outcome (ie her reprimanded if not sacked and a written apology) I would be taking it further to the governing body.
is it possible her son overheard her talking to another school teacher/assistant? If he overheard there probably isn’t much you can do however she definitely would need a talking to to be more discreet around children who could hear.
while ADHD is nothing to be ashamed of and some pupils would be happy to tell others, it’s the lack of control and it being told without our consent that is the issue here.
I don’t know if I’d want the boy in trouble exactly (as he’s taking his lead from his tactless mother) but the teacher should tell him (nicely) to not share info like that if he overhears it again, as it is wrong and can be upsetting for some people. If your son doesn’t want him to get in trouble though, that’s understandable.
I would not tell your son that you plan to complain about his mother.
I would email the school your complaint so that you have a paper trail as it were.
unfortunately I never told my parents at the time and I bitterly regretted it.