It doesn't sound like you even took him in...??
What did the vet say? Why does he have an infection? How does he think it got infected?
I know you already said you wanted to give him an injection... so I hope you really took him into the Vet-
If not you need to- I think you should take in the Chi too--
My coworkers dog ( small 6 lb chi mix) got pregnant on accident and she passed away. It is nothing to take lightly.