Not sure how common ticks are there, but Toronto is at least on the same latitude if not south of here--here, a combination of low platelets and WBC is almost always an indication of tick-borned disease. In particular, that pattern is present in the cases of anaplasmosis we've experienced.
Might be worth having him tested for it, given that he lived the first few weeks outside during the summer.
Treatment in dogs and people is with doxycycline. Not sure if they also treat cats with it.
I hope they figure out what's wrong with Willy quickly and he recovers fully!
And welcome to the board!