Is it possible or likely that he was ethnically russian living in modern-day Poland? 

The whole orthodox thing with the patronymic really confuses me. There was any massive campaign of russification in the region to justify that?
-no. thet just weren’t polonized.

Anyway, you’re brazilian now, but with partially east-slavic origin. Surname of your grand “boyko” is ukrainian by origin, but this sunetnicity is younger than 100 years.