Hi YorkieProud!
Please take a look at this website, it has lots of info on many brands of dog foods.
Dog Food Reviews | Dog Food Ratings Beneful is one of the worst foods out there.
Beneful Dry Dog Food | Review and Rating
there are many super premium brands available at Petco or you can order on line. Also look in the health and diet section and look at the stickies on top and you can ask for free samples of food!
Yorkie Health & Diet - YorkieTalk.com Forums - Yorkshire Terrier Community
I also want to tell you, my foster yorkie had bladderstones, and I believe it was bc her former owners were feeding her Beneful. She was first diagnosed with UTI which was not well treated, and she was not given her prescription food and medication, but instead continued to feed Beneful. That is how I think she developed the stones. These little guys eat so little, it really isn't that much more money to feed them to good stuff. Only the best for them right?
As for beefing up your little guy. I had the same mindset as well. My Uni is not as small as Dawson. She is naturally 5 lbs but very tall and skinny/bony. I wanted her to get to 6 lbs, so I found a food she liked and overfed her. In a short period of time, she did gain weight, but she also ended up developing mild pancreatitis (which she is still recovering from now). If you guy is happy and healthy, keep him the weight he naturally is at. It's better for their joints and knees anyway for them to be a little on the skinny side. He is energetic so I think that is his ideal natural weight.