"See you soon!"
Two factors at play here I think:
How the word is stressed - because the stress is on the first o in the actual stem of the word, the other os are unstressed and sound more like a. I'm trying to think through examples in my head where you would put stress on this since it's a genitive case ending, and I don't know of any.
For the ого (and его), really any time you see it that, it's going to be on a masculine genitive thing and the "г" just sounds that way, not sure of why. I imagine it happened as part of natural speech since it's just easier and faster to say.
So in short, as I think through it...ого and его will just always be this way when you see them - someone correct me if you can think of examples that differ, but I am not able to right now.
Wow, this language is blowing my mind... I already knew a good amount of Spanish, and that has helped me pick up Russian faster, but it is amazing how these languages translate to English. I want to understand everything between. I dont know why, but I really wanted to learn this language