Love the idea of the magnet used to retrieve the key to his locked cock.  But we've ignored those tits of his so far so maybe instead of the string in his mouth, the string could be tied to the chain between some nice clamps on those tits.  He would have to bend over at the waist as cbr998 says and the string could be on a ring that the tit chain goes through so it can slide as the puppy maneuvers it.  And maybe put several keys of the same type available at his feet and for every wrong key he selects with the magnet, he gets the added pleasure a different penalty.  And if you are really in the mood for a frustrated puppy, each time he selects a wrong key, rather than eliminate that key, you scramble them up and place them all at his feet to try again.  This is getting to be fun!  Now where did I put that TARDIS for him to use.