Yes. Right indeed :)
If we're talking magnets how about a magnet on a length of string which he holds between his teeth. The string just long enough so that the magnet - if he's not careful - will attach to his cock cage. The key can be placed on the floor in front of him but in an awkward position so that he has difficulty getting seeing it without moving his head to the side making the magnet swing about a bit. he can then lower himself to try and get the key without catching his cage first. To make this work you need to be tickling him with one of your devious items all the time he's trying for the key. The prize is up to you if he succeeds. If he laughs he's likely to drop the string.