He said that the boy was dating not in order to get married.
You’re making it sound worse than it is. He said the boy has commitment issues, not that he is stringing people along intentionally.
There’s no way Abba_s could know the real reason. It was obviously, even if worded wrongly, merely speculation.
Ordinarily, motzi shem ra is saying something negative and false as fact. Do you have a source saying that speculation falls under the issur?