I'd definitely wait until he's better.
Like humans, our little Yorkies are able to recover faster from a surgery when in good health. When we or dogs are sick, our bodies are already battling something awful. The body needs to rest and recover. Just picture yourself fighting a stomach flu (or something worse), then undergo surgery, and have to recovery from whatever sickness and a big surgery.
And for Yorkies, it's even more crucial they're in good health, because after all, they're only a few pounds in weight!