Make a timelocked transaction that spends his coins when he's a 100 years old, or a 120, some time that he obviously won't reach.
For practical reasons it is even better to create timelocked transaction with something he could live to, like 1 year and then just spend those outputs at least once a year to a new address from which he can create a timelocked transaction again.
As I said, I think GreenAddress wallet already does this automatically for you.
I am not sure if they are open source, they have a lot of repos on their Github page
https://github.com/greenaddressPrematurely broadcasted timelocked transactions are invalid and ignored by the network. That's why additional application logic is needed to broadcast the timelocked transaction after the timelock has passed.
They can't be included in a block until the timelock is reached, but I do assume that they stay in the mempool for a while.
Still, it makes since that you should keep your wallet running at least until few days before you die, so it doesn't disappear from the mempool. Most wallets, including Bitcoin Core, will keep broadcasting your transaction until it is included in a block.