If he's not seeing the game, it's likely because the game isn't "fully" released on Steam yet. That's why you can't find the game in the main Steam store either.
You should however, be able to log into your Steam account on his mac, and download Universe Sandbox ² to his computer's hard drive using your account. After that, even when you log out of your account, it should be visible in his library. He can then request access to it through the family sharing system.
It should just work once the game is fully released, but give that a try and let me know if it works out for you.
In regards to Xriqxa's comment, that's referring to games that use third party DRM only. Universe Sandbox ² does not use third party DRM, it only uses the basic Steam Authentication.