I like what Judi Z said. He's probably punishing himself enough, knowing that both you and his principal know the content of the note. I suspect going to a specialist is not necessary. Our son's school covered sex education in 6th grade and I've talked with our 12-year-old son on and off throughout the years. I think its great you and your husband are going to talk with him together.
Gloria V has some great points, too. I like the premise of wanting to hear it from your kid first, rather than from the school first. (I haven't said that explicitly with our son, but that's basically what happens.) And never write anything in a letter that you wouldn't want the world to know. Good rules to live by...whether it is a handwritten note, an email, or something on a web networking site.
I might approach the conversation from the standpoint of
1) What was his intent with the note?
2) How does he feel now that it is "public" what he shared with his friend?
3) Passing notes in class is disruptive (getting in the way of class learning for him and his friend) and disrespectful to the teacher
4) Sex is part of life and can be fun and full of joy and respect if used properly
5) If handled crassly, sex can be negative which isn't productive in the end
6) What has he learned from this experience?
7) Blah, blah, blah...
He may just end up hearing the last one (7), but you know him best. If you think it's a relatively isolated incident and he's basically a good kid, I would have a conversation but not make a huge deal out of it. If you are concerned, or learn, that he is exposed to some influences you'd rather not see in him, talk about that with him. Explore what expectations you have for him and what he expects in response.
Kids his age are often experimenting with boundaries and learning what is acceptable and what isn't. Now is the time to explain, to the best of your ability, where the line is drawn for you and, you hope, for him.
And don't forget a little humor in there. He may be mortified by all of this. Give himself room to laugh at the situation. Not laugh it off, but chuckle at his predicament and what he has learned from it and what he still has yet to learn.
Good luck!