Growly, I don't see you as expecting perfection here. As others have said it is perfectly reasonable to expect a therapist to remember your name and not to put your name down for groups you haven't agreed to.
I also want to say that really, it doesn't really matter whether his forgetfulness is caused by sloppiness or MS, or anything else.
What's important is that you feel you are getting the service you deserve, and are paying for. Managing his medical condition is his responsibility, and you don't have to make allowances for it. So if it isn't working, you are well within your rights to look for someone who can meet your needs.
To be honest I would find Sparky's comment about expecting perfection a little patronising, especially considering you really aren't asking for perfection!
I hope you are able to find a solution that works for you, but remember, take your time, put your needs first, and no decision is irreversible.