I don't think you would be crossing a line by inviting him since he has the responsibility to set his own boundaries. I guess I am different than some on here, we all have different relationships with our therapists, but I would never want my xtherapist to come to any event of mine like a wedding or anything. What if he drinks too much and ends up singing karaoke or something, lol, what if he starts flirting with the bridesmaids, what if he cries, 'sobbingly' and loudly, through your wedding vows, and do you really want to see your therapist dancing? Not me. Nope, not me. Talk about an idealization-crasher! hahaha

Seriously, though, it is between you and your therapist. Like others said, prepare for him to tell you no.