Eventually the government is going to come after him. If you just happen to be sitting on 400 million without paying taxes the government is going to be interested.
That simply isn't true. As someone else said, taxes are not owed on bitcoin until you realize your profit by converting them to USD or spending them on goods.
He might have $400m worth of bitcoins, but can he actually sell them and get $400m for them? It would be tricky.