If he was from a village, then he was most likely Belarusian. Belarusian were eastern Orthodox and russified especially when it came
to writing. Boyko is common western Belarusian surname. Boyko is
not a Russian surname. Belarusians used patrynomic names. Few Russians lived in western Belarus / eastern Poland (Grodno governate) during those times, especially in rural areas. In general , Poles were Roman Catholic. Potrubowszczyzna village is 20-30km from Grodno city , north-western Belarus (border of Belarus, Lithuania, Poland) which is too far from Ukraine. During 1921-1939 western part of Belarus was part of Poland, so your ancestor traveled using Polish official documents and he was Polish citizen. Also, north-eastern Poland has Belarusian communities to this day. So , it’s likely your ancestor was a Belarusian.