I tried using the healing potion to cure my boy's infected injury, but it won't let me use the potion on a "healthy" wolf. Is there something I'm missing?

You'll need a to use a red healing potion

Ah, ok. Does it have to be crafted?

You can buy it from barter in the marketplace, under the Relem tab :)

But yes, it is also craftable

I believe you can also use a robin feather, which tend to be a but cheaper