If your dad is not bulbar onset AND if he is able to eat normally, I strongly recommend walnuts. They are full of beneficial fat and are considered to be antioxidant.
After my husband's weight plummeted from around 200 to 169, I started giving him 1/4 cup of walnuts every morning with his oatmeal, banana and prune juice. He quickly began gaining weight. At his last weighing he was 197 pounds.
I also give him a can of Ensure Plus (shake) every afternoon. Basically, fats have been found to be neuroprotective, so forget all the "wellness" advice you ever heard and give him fatty, comfort food instead. Another easy way to get "good" fats is to cook everything in olive oil. Avocados are also beneficial.
Good luck.
VL