I am looking at the Game Pass Ultimate, for $15 a month it doesn't seem terrible since it includes your Live Gold payment also which would be $60 a year.

I have two questions:

- If my son shares an account but uses another xbox console, can he also access the Game Pass or does he need another account?

- How often does a game get dropped, because I see that if it falls off the list, you cant play it anymore which would piss me off pretty good to be in the middle of a game and have it drop and then I have to buy it.

Your son would be able to play on another Xbox but two people can't play under the same profile at the same time, so you have to make sure that if the two of you want to play the same game you'll need to start it under different profiles. Several different profiles can access you're game pass if you add them to your Xbox family.
I'm not sure how long the games are available. Some are there for several months. If it's an open world rpg that can be played a hundred hours you may not get through it in time.