Well like the old saying says 'if you wanna be 10 lbs heavier, you have to eat 10 lb heavier'- Something like that. He's going to need to eat more if he wants to gain weight. A weight gainer is fine but he can't be drinking it 3-4 times a day! Very expensive.
I recommend ON Pro-Complex Gainer or CytoSport
Cytogainer.
Do the best you can to get your dad to EAT, it may be hard but I'm sure he can do it, unless any health problems come into play.
Also, make sure he takes the weight gainer correctly and has a proper training routine.