As has been suggested its a matter of trial and error. 3 weeks is very early for anything to be fixed but its not too early to try. Tastes are weird, strange cravings, and the connection needs to be made between the brain and the mouth. As Paul suggests visualising food is very helpful. Its also go for foods that are always the same. Eg processed meat like a hot dog. You might put different stuff on it but the meat of every hot dog he's ever had tasted exactly the same. Take away foods, like KFC mashed potato always tastes the same. Buffets are good for experimenting with small amounts.

He needs to be patient and understand the mechanics of whats going on here. The body has been through a massive trauma but the signals his brain get tell him the food doesn't taste right so must be bad. It hurts to swallow, the fear of the pain can take you a step backwards before the food makes it to the mouth. And once the food is in there, a lack of saliva can make even soft foods feel like you are a cow chewing a cud.

Experimenting is good for testing the boundaries as things improve. Bottom line is he needs the calories in whatever form he can take them. The luxury of eating something that doesn't taste like stale phlegm comes later!