imessage shared w/Apple ID - who can see?

HELP!

I just realized that my Apple ID for imessage was linked to my husbands e-mail account. I have just now set up my own Apple ID which is NOW linked for imessage and icloud, but prior to that it was HIS e-mail in my imessge setup. Meaining, under Settings/imessage/Receive At, it was MY phone number but HIS Apple ID and e-mail.

So - here is the problem. I am planning him a surprise B-day party, and have been going back and forth with a caterer and his friends/family, all sorts of people. Does that mean that when I am imessaging and texting with them that he has been seeing these messages all along? Or, if I sync with his laptop (the primary device) has it been dumping all the messages to his computer/phone? Is there ANY way he can get to those messages?

And truthfully, I have been kind of frustrated with him lately and been saying not the nicest things about him to my sister over imessage (bad wife - but not every marriage is perfect). But, I would really like him to not be able to see those as it would pretty much take away from the nice party I am throwing...YIKES!

Desperate for some answers! Hopefully someone can provide some help...

Thanks!!