I miss my moms ex-boyfriend. How do I tell her?

A little background because I know this sounds crazy:

My mom and her boyfriend had been dating for 13 years before he left. He's been around since I was five and I basically considered him my dad (my parents have been divorced since I was 2). This was the first year he wasn't at Christmas and it'll be the first year he isn't at New Year's <a href="https://play.google.com/store/apps/details?id=com.glow.android.eve">Eve</a>. I know that they are still talking because I see his name pop up on her phone screen. I miss having him around (my father isn't exactly the greatest man).

Now why he left:

He's a pilot, a first officer. I helped him study so he could become a captain. During that time, he had to basically live in Dallas for training, so we didn't see him much. But when he got the captain position, he stopped coming around even more. My mom then told me that he left, but I don't understand at first. What happened was the base for his airline had moved to Chicago, so he wasn't home very often, maybe 1 weekend a month. My mom also said that they were having fights now due to his new salary.

Can anyone give me advice on what is happening? Thanks!