Introducing our parents

My birthday is coming up and I invited a big group of people to go out to dinner including most of my family and parents. I mentioned to SO last night that I'd like to invite his parents as I've formed a bond with them. He says he would prefer not to introduce our parents yet. He wants to wait until we are engaged. We've been dating about 8 months now. We are planning our future together so to me I don't see what the big deal is. What do you think? To soon or no big deal?