Try malted milkshakes. Until I turned 42, I was underweight but I've been told malted milkshakes are great for putting on weight. They're given to cancer patients who have lost a great deal of weight.
That's true; when my mother was fading away with cancer, the Sonic malteds I'd bring her would be the only thing she could get down and keep down.
Unfortunately, I'd pick up one for myself at the same time....