He has a girlfriend. I'm not sure what their relationship is like.
Girlfriend. Is that how "he" refers to her?
Not...significant other, soul mate, partnered, wife, engaged?
Girlfriend is fair game in my book...and sometimes it's a smoke screen to hide behind.
Just joke and ask him if he has a brother or a twin, because you would enjoy spending time with someone just like him...and leave it at that for now.