Avocado, butter, nuts(pecans, almonds, walnuts, macadamia nuts), cream. Those will provide the needed calories, and along with enough protein, to build the muscles, his weight should come back.
I lost lots of weight at first too, but as long as he's getting enough calories I wouldn't worry about it. I lost 60lbs initially, but have put back at least 10 in muscle mass. I know it's muscle mass because my waist size is as small as it's ever been.
