A prepaid Bitcoin gift card isn't that kind of useful because you could just send some in his wallet as a gift. It could be applicable with a design but the thing is it could not prove its usefulness. A disposable wallet could be great where that will be your temporary wallet as of the mean time, but prepaid gift card where you can put it in your wallet is not that practical because the Bitcoin transaction itself is continuous and it is processed by the miner. Unless there is a server in the Bitcoin network that holds all these prepaid or credit funds that will be enabled once the prepaid code is activated with the use of internet, but it still all leads to the same concept that you're sending it to someone's wallet where you can just do it simply using Trezor or Ledger as it is a solid wallet for Bitcoin and any other currencies.