Sounds dangerous to try to figure this out on one’s own.
First, you have to determine if he really is a rasha, and I don’t think your average yid-in-the-street is competent to do this. You would have to ask a posek, and he would probably ask for proof, not just something like “I heard that…..”
Second, you would have to ask a sheilah about lashon hara. This is a pretty heavy kind of issue, and again you need your local posek.
Third, you would have to be clear that you are really only doing this l’shem shomayim, and that there aren’t any personal issues involved, or that you’re doing it eacause it’s giving you hana’ah to denounce somebody else’s wrongdoing.
It all sounds pretty perilous to me. If you can’t get this person to make teshuvah by speaking privately with him, consult your posek. Under no circumstances try to act as judge, jury and journalist and talk in public about it without getting a reliable halachic psak.