The opportunity to pack him full of high quality proteins and nutrition right now is something that he should take advantage of and you encourage. Not empty calories for calories sake to put on weight, but a calorie dense regime that is nutritionally sound. When he moves into the world of radiation, after a couple of weeks he will likely hit a wall. That is where the desire to eat will wain. Being ahead of the game will be an asset. Watch his habits, and have him keep a daily journal of meds and food intake by type and calorie. When you see a trend that this is dropping off, will be time to consider options. But be careful of allowing this to go on for too long, especially to the point of under 1000 calorie or worse days in sequence.