Is it safe to add supplements to my grandfather’s food to help him gain weight?

My personal opinion is that this is an excellent approach. You should, however, check with his doctor in case there is some nutrient that interacts with his medications. Many elders lose their appetites and we have to temp them with what we can. I found fresh fruit was helpful for my mother. Keeping it light and easy to eat is important. Also, keeping portions small if you serve several things, so they don't feel overwhelmed, can help.
Good luck. You seem to have wonderful instincts.
Carol