My mom has a "boyfriend". They're both in their 60's and have been together for 22 years. We're a little past class rings and going steady here. They're not married and neither one of them have any intentions of ever getting married again - they both have been been through that one multiple times and came out the worse for wear. My kids have grown up with the concept of him, but sometimes it's a little awkward. I can tell you that he doesn't consider them calling him "Grampa" a good option, though now that he has grandchildren of his own I think he's mellowed somewhat on that one. I just call him by his name. But where I run afoul is how to describe the relationship. If I say he's my mom's boyfriend it just sounds weird. But partner or signifigant other or whatever just sounds pretentious.
Adult children is another one our common usage really doesn't handle well. I've been accused of being a hypocrite for talking about playing M rated games with "my son". It was one of those friend-of-a-friend things through church, and the friend had talked up my game writing to this person but didn't say much detail about my kids. The son I was talking about is 24, and has served a full hitch in the Army as a Ranger. He's a combat veteran. I doubt they've put much on a disk he couldn't cope with (probably better than me) by now. But when I talked about playing Mass Effect 3 with him and I said "son" she assumed he was seven or something and sorta came unglued.
Other languages actually build concepts like this into them. Japanese, for example, has the word "tomodachi" which usually translates as "friend", but "ruitomo" translates as "friend because of shared interests" and it's used for friends who are also co-workers and school friends and the like. They use "ryouyuu" for a coworker you don't consider a friend, or "kouyuu" for a schoolmate you don't consider a friend. I'm not sure building in that kind of complexity is the answer.
I'm not sure there is an easy way to go. I'll just have to stick with the awkward and just throw myself on his mercy. Mom and her boyfriend are here Outside and I'll be seeing them here in the next couple days. Hopefully he'll think it's funny.