If a man had a child with his ex-wife after she had relations with another man, besides being assur, is the child considered a mamzer? What is the status of the man?
Essentially such a question is not for a website to answer, because there are other issues that may have to be dealt with here, (since she was together with another man, we may not know who the real father of this child is, and would have to be discussed personally with a Rov). However regarding your specific question, the fact that she had relations with another man (assuming that her get was valid) will not make the child a mamzer. The status of the man remains the same, but he has to do teshuva for what he did.
Shulchan Aruch E:H 10-1.