Yes. I think it's possible to move past it and I think you should talk to him about it. Because what I would guess is that he followed those accounts by accident, or someone hacked his account and followed those profiles to get their following numbers up. My husband had his account hacked once and it took him forever to figure it out because he doesn't use Twitter much. It doesn't make sense to me that a therapist would follow those accounts publically under his own name. It's so easy to cover your tracks if you really want to look at porn. So I bet it was a mistake made by someone who isn't paying attention...as in he was hacked and didn't realize it.

If it were me, I would say something like..."hey I saw that you had a public twitter account and I wanted to see if you were following anything that would be helpful to me and I noticed these really disturbing accounts and I wondered if you knew they were there." To me twitter is different than facebook. People follow strangers all the time. Seriously, don't let this ruin a relationship that has been helpful to you without at least talking to him about it. He should be grateful you told him about it because it affects his public reputation.