Does it mean anything? Not really. The most intense fantasies I have are of girls I ALMOST slept with but didn't, or just had a one-night stand with. I rarely think about girlfriends that I had for years... which is strange but I guess I "had my fill" and don't have to imagine anything.
Online porn is artificial and repetitive (just my opinion). Not only do they leave their shoes on, they rarely kiss or show real emotion. Memories of your past become amplified as you get older and for myself, it's those old love situations that turn me on the most. There's a personal connection and time makes the fantasy better than it really might have been.
It's a bad idea to get upset about this. He'll just take it into the bathroom with him and be afraid to be honest in the future. Getting upset about something that isn't really happening doesn't make a lot of sense either unless.... he wants to still hookup with this girl, which he most likely wouldn't do for risk of destroying the fantasy.
I never look or think about porn when I'm masterbating. I enjoy it as eye-candy or as a way to get aroused, but it's always girls that I've known in the past that are in my mind when I'm rubbing one out. Chances are, in the future your bf will be thinking about
you like this rather than porn. So give him some good memories.