I wouldn't use the Science Diet IMHO. It is not the best of foods. Do a search on here and then make your own decision. I have the same problem with Lily who needs to gain weight. I feed her Merricks canned puppy plate. She is so picky and that is just about the only thing she will eat. I also add Rebound which is a liquid diet supplement for dogs.
feminvstr posted the following recipes for weight gain. I have not had a chance to try them yet.
http://www.yorkietalk.com/forums/sho...t=71553&page=4
She also said to add avocados to help them put on pounds and mix in a little alfalfa into their food.