I don't live in the us but my son was born there and my sisters ex keep filing taxes for my son and the thing about it I asked him to stop and he told me to call the IRS and tell them, now only to learn that he hIS BROTHER IS NOW DOING IT how can I stop them

Report it to the IRS. They probably feel they can get away with it, because you don't file a US tax return.

Agree with Scott, I will "assume" since you did not say, that your son lives with you and does not live in the US.

I guess the question is how do you know someone is claiming your child if you are not filing since there is no notification sysem,

How do you know who is doing it, are they telling you ?

But yes write a letter with documentation and mail it to the IRS