It's possible he's been confronted, or even charged with something, but he's not going to be locked up or banned from the metro for this behavior. If he doesn't care about his record, he'll keep coming back to do this.

Are you texting MTP when you see him on the train or after the fact? I don't know if you can attach photos to the text line but at least provide a description of what he's wearing, what stop you're at, which direction the train is headed, and car number if visible. I've had luck with MTP getting officers on the train within a stop or two.