I used to do that, too! Of course, there are a lot of reasons it could be, so asking him would be the best... as long as you let him know that no one minds him doing it, and you're just very interested, maybe he wouldn't be embarrassed. :]
When I used watch myself as I talked, I was mostly watching my mouth. For me, it was strange to hear myself in one place and see my mouth moving those same words in another place. I also liked that I knew what words were going to come out of that mouth (my mouth, I knew, but the reflection made it visually separate from myself, for me) so it had a soothing, hypnotic feel ... it gave me something solid to focus on. Besides that, I also just like how reflections look in glass (mirrors are unremarkable...) They can become transparent and ethereal, or shadowy like in a sidewalk store window.
So, I mean, there are three reasons right there just for one person... so ask him! His reason might be something way more interesting~