It seems like this is something some therapists would do. It isn't a social event and it's definitely related to your therapy, so I don't see any ethical issues. If you want him to go, I definitely think it's worth asking. I don't know if he would be willing or able to go since he presumably works on weekdays, but it does sound like he offered before, so I would tend to take him at his word. If it were me, I might ask via email just because I know I would be nervous to have to wait while he thought about it and responded. That also gives him time to think and to consult other people, if he needs to. I don't know that there is any way to minimize the potential pain of a no. But if you don't ask, then you know that the answer is already no, right?