Well, he is handsome! Sorry about that. Been there and done that here on my little suburban teeny weeny farmstead! A few roos have come and gone QUICKLY!!
Yep, definitely not a RSL hen but an EE cockeral. As someone else said, it has the wrong leg color and comb for a RSL. It also has muffs which RSL do not have.
As for who its Mom is... chickens will hatch out eggs that aren't theirs. It's very possible that its genetic mother isn't the same as the chicken who hatched it out.