Those dental diets are a big bunch of hooey anyway, hun
Could you stomach giving him a RAW chicken wing/bone 2-3 times a week? This *REALLY* helped with dental issues with my chis.
As far as diet goes, please get him off that awful food...I don't want to sound dramatic but foods like that are basically "garbage" and even can contain people's euthanized pets. The thought of it makes me sick :-/
I would find a specialty pet store in your area and see if you can get your hands on Innova, Fromm (fourstarnutritionals.com), Acana or Orijen if kibble is what you are comfortable with. These foods are a bit richer and MUCH healthier than what you are currently feeding and will help a ton. You might be surprised that the price tag is a bit higher but you 1) feed less of the good foods and 2) the health benefits are unparalleled.
If you really want to go for the "ultimate" dog food, try Ziwipeak or The Honest Kitchen -- both are quite expensive, although I would imagine it's not too big of a deal for one chihuahua.
I would also add a salmon oil to his diet as others have mentioned -- this will really improve coat a lot. Petco has a salmon oil they sell or you can find them online.
The missing link is a decent supplement but it MUST be used in conjunction with a quality diet. If you ask me, though, it is overpriced. When you feed a good diet, you really don't need that much of a supplement.