If you assume they won't be able to break the encryption or coerce him, then you should buy BTC now. If he ever gets out of prison, maybe he'll be able to unlock a backup copy and spend them, but that will be 20 years down the road.
I think another important question is whether it will help his plea if he gives them the keys: He appears to be just an average shmo (no surprise) and I'm sure would be happy to give away his privkeys for a few years off of his sentence.
The other question is then whether the FBI would then liquidate the coins via an exchange, or just sit on them.
