I'm in awe your son has this kind of support.
I'll echo Candu on the forgiveness issue. When/if it comes, that's waaaay down the road, imo. I've never forgiven my deceased abuser, nor do I have any inclination in that direction. The parents whose abusiveness set me up for the perp - though I now completely understand the dynamic, family history, etc. - there's no forgiveness for them, either.
Sometimes, too, there's a point when the current therapist has outlived his/her usefulness. That's fine. Your son needs to understand he's under no obligation to this therapist. His recovery is about HIM, not the therapist. It's about the healthiest selfishness I can imagine.
One suggestion I'd make may be hard for you to hear as a parent in this circumstance. But it's made a considerable difference in my progress. That's anonymity. As an 18-year-old, I'd have a hard time being frank if I knew a parent was looking over my shoulder at what I'd posted. His location, DOB, etc., are not things that are required on MS and, for now, I'd discourage him from providing that or any identifying information online. I'd even go so far as to tell him not to supply you with his MS online handle.
Alternatively, he should know that PMs are another option if he has heavy stuff with which he's dealing.
That said, I don't know your exact situation and I recognize that cutting is an issue. Obviously, though he's reached majority, it's your call.
Well done...and I like the French fry quote!