I don't think Bill will invest in Bitcoin, or at least publicly. Im convinced he owns some, not a serious stack (by his standards), but im sure he owns quite a bit compared to all of us. Hell Microsoft accepts Bitcoin, and he has talked positively about it, so im sure he got some. But making it public? Not sure if it would benefit him. Bitcoin has some anarcho-capitalist like features that he doesn't enjoy.
Bill Gates never reveals his real political views, but we can look at his actions.
He's not a fan of anti-trust and anti-competition laws.
He doesn't respect intellectual property/patent laws.
He doesn't seem to give a lot of aid money to African governments but he privately helps Africans with his private foundation.
It's likely that he holds some anti-government views but he's never going to admit it because it would hamper him professionally.
I agree with you because I also think he owns bitcoins and is funding bitcoin research and development or at least exploring ways to use bitcoin. But he simply can't accumulate 1 million bitcoins or anywhere near it without sending the price skyrocketing. The more he buys, the harder it gets to buy. Satoshi has a giant headstart and it's only going to get harder and harder to catch up to him no matter how rich you are.