His mom was in a deathbed for a good amount of time so he probably learned to play just to comfort her. That's not a stretch at all since he decided to become a merc to pay for her medical needs (attesting to how much his love is, for his mom). Piano is one of the least expensive forms of entertainment. His accent is a stretch for his character though.
Also, his mom was obsessed with Wesker. So she probably raised him up to be a smarty pants; and learning the piano at a young age is definitely on the agenda (just ask any parent with "high expectations" for their kid, I guarantee you they would have raised their kid on some form of instrument)