I think it's totally reasonable to ask. I'm not sure if he uses the same email for clients and his personal contacts, but if not, that may be part of the issue. He may not check that email during the weekend. If that's the case then he probably won't change his ways but it certainly can't hurt to ask. I also agree that you should not see this desire for contact as something to be embarrassed about or as fodder for discussion in therapy. If your therapist can't offer weekend contact, they should make it clear it's because of their own scheduling reasons and nothing more.