There hasn't been any casting call for actors to play Silas. Now I have a theory, but it could be wrong.
Jeremy is the first person fed upon by Silas, whose body seems to have fossilized. What if Silas assumes the appearance of Jeremy since his was the first life essence that he took upon awakening?
This would explain why we haven't heard anything about an actor being cast to play Silas because Steven McQueen himself would be taking on the role.
Silas is a master of illusion as illustrated in the last episode so it wouldn't be that far off to assume that he would take on Jeremy's appearance to confront the Mystic Falls Gang.