Thanks for the help. Tonight he's getting nothing but Wellness with a little bit of pumpkin mixed in.
Artie was never in a shelter.
I'm a security guard, and a guard I used to work with called me when she saw Artie running around the grounds of the factory she guards. Artie was about 6-8 weeks old at that point, and neither the vet nor I believe he was outside more than a few days, given the lack of fleas or ticks, both of which are common to the point of infestation in southern Ontario in August. He was outside long enough to get dehydrated in the intense heat, but not long enough to get seriously sick. Thank the fates for Jan and her observant nature. And for her knowing I can't say no when it comes to a critter in trouble.
I'll call the vet in the morning to see if they looked for Coccidia in his fecal sample. I have a vet appointment on Friday for one of my rats.. I'll see if they'll let me squeeze Artie in there too.