Cock & Ball

Keeping the cock and balls correctly place can also be done with binding. Securely placing the rope around his balls, with part of the cord used to separate each ball. The remaining cord is then used to lash down his cock, both keeping his meat firm and secure.
You have the further options of allowing him a method of humping himself  (normally the only allowable way for a dog), you may also keep his meat in position should you wish to  keep a catheter (piss or electrical) it will lessen the chances of it coming loose once in place.

If you want your dog to have biggy doggy balls. Then putting on one of these will not only remind him of the reassuring grip of his Master around his balls, but they will hang correctly.

I only know of WildCat and Robs (see suppliers) that provide these - I an sure other perv shops can source them. They are all HEAVY, but they come in different lengths, and maybe stacked when you have increased the initial stretch. The 2 screw option here (Robs) ensures it is dead tight - but it a very fiddly thing to put on. GREAT care must be taken when putting it on so as not to crush and of the ball sack when fitting it. The slave should not be gagged.
The easiest way in fitting it that I have found, is to get both screws started, then close and rotate the ring ensuring that there is not thing trapped, then hold it tightly together and screw one of the screws firmly home. This therefore means that the other version a peg and one screw is easier to put on.

Life is much easier if the slave is fully shaved.