Honestly, the cheapest and most reliable method is to buy a cheap 50-60 qt cooler and take it as an extra checked bag. I would take a roll of packing or duc tape to the airport, and wrap it a couple times when you check it in (sometimes they want to peak inside). If the fish is frozen when you load it, it should still be when he gets home. If not, use a few of the freezable ice packs to help keep it cold. Regular ice and dry ice is not allowed.
Done it many times and never have an issue.