Welcome to YT.
There are numerous people here who home cook for their yorkies so I'm sure they can offer more about his diet. But yes, chicken and sweet potatoes can be nutritious but you may need to add a bit more to balance his diet out, -- not sure how much kibble you're feeding, so you may actually be ok on that.
There is a thread here with all kinds of homecooking ideas and recipes
Are you referring to Breeder's Choice Pinnacle Canine Chicken & Oats Formula?
I believe that is suppose to be a pretty good food with a 4-star rating from this dog food analysis site:
http://www.dogfoodanalysis.com/dog_f...ndex.php/cat/1
That's a good place to get info on dog food - yorkies don't need the 6-star foods which are mostly high in protein, a good 4 or 5 star food is good.