After partitioning Lublin and Chełm belonged to Austrian Habsbourgs between 1795 and 1915, then to Russian tsar. After WW1 region became a central part of regained II Rzeczpospolita Polska (Second Polish Commonwealtah).
would he then be classed as russian or polish
Of course not. Ethnic nationalism was a very new invention that time and (with exception of higher class) nobody knew what's going on. People were classed mostly as Catholics, Orthodox, Old-Testaments, etc. You have to remember that i. e. for Habsbourgs subjected peasant from Cracow region "Pole" meant enemy even in 1900 because Pole was considered as an oppressor belonged to noble class "nation". Earlier, in 1846 almost 2000 nobles lost their life for that. II RP didn't class people according (new fashioned) nationality too.
Before WW1 people from partitioned Polish territories were called up to join all three occupant armies, but they were not sended to fight with their countrymen.
After WW1 people called up to join the new Polish army were sended to the other former occupation zone, i. e. from Russian part to western border, from Austrian part to eastern border, etc.