I have a similar issue with my 3x great grandfather who was in the 97th Foot and also served in the Crimean War. Prior to that conflict, my 2x great grandfather was born in early 1853 and unless my 3x great grandfather was on leave from where he was stationed in Nova Scotia (from mid 1851 - mid 1853), he cannot possibly be his natural father! He didn't marry my 3 x great grandmother until Oct 1855 on his return from the Crimea, after another son had been born earlier that year. This one IS likely to have been his biological son, conceived when the regiment was back in England before being sent to the Crimea, but sadly he died in infancy.