I've heard of people sneaking in the ivermectin by hiding it in a little cup of chocolate pudding, or applesauce, or something similar that they can tell the nurses their loved one likes and they're just trying to cheer him up. Might hide the taste of the paste, too.

If can get hold of the ivermectin tablets, you could probably crush them to a powder and also hide them in the pudding/applesauce.

Good luck. Your dad is lucky to have you looking out for him.