I hope he doesnt know all your customers and prices!
Dump him, quickly! Then let all your customers know so he cant go round either collecting the payments from your clean, or cleaning them a couple of quid cheaper.
Dont feel bad about it as you will still be teaching him a lesson
at the moment he feels that this work is easy enough for him to do so show him the door and let him learn;
1) not to bite the hand thats feeding him &
2) it aint easy for him to just break into without help.