need some input

[newmoon]

i work with my bf of 1.5 years. we're ok - we get along some days and not others. i don't see forever with him and i think he feels the same. he likes me but it's not forever. there is a 10 year age difference, he is older. we also work with a girl that is younger than us both - 17 years younger than him and 7 years younger than me. i can literally see the chemistry between my bf and this other girl when i watch them together, even though she has a long-term boyfriend too. but, there is just something 'there' - he refers to her in conversation sometimes when we are alone outside of work, very casually, but her name comes up, and he has started to say things about bigger/fat women and how they are supposed to be great sexually (she is very large and i am very thin). he mentions how nice her new car is, the color and model and how she makes such good choices, how nice her hair is, how nice her scarf looked at work, how badly her boyfriend treats her, etc. just little things. and he says things about how good it feels when she teases him and jokes with him, like little kids. i told him the other day that he's old enough to be her father. i haven't told him to stop mentioning her altogther (for fear of looking jealous), but i kinda am. i'm not sure how to tell when a guy is really interested in someone other than his partner, or if she is just some infatuation he has. i cannot see him/her being together because of the 17 year age difference, but who knows? i just wonder if i should mention anything and if so, how - i don't want to look insecure, because (sadly) he's the type of guy that would do it more if he knew it bothered me. i should also mention that even though we've dated 1.5 years the office workers don't know, so i've been tempted to tell this girl, or others, so he might stop being so close to this girl