Ahhh, he is a cutie! I rescued a two year old yorkie (he's in my avatar picture). He weighed around 12 pounds but was skinny and under weight and had health issues that we were unaware of. Now 14 months later he is healthy and tops the scale at 15.1 pounds and is a sturdy muscular little guy. His weight is perfect for his body size and frame. Your vet should be able to tell you whether he is a good weight for his body frame, but I suspect he is just fine.
Diana