The male attribute is a fine object for bondage, as it have a wide selection of possible points of attack ...

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One option is to tie a ( not too thick ) line around penis, just behind the penis head. This could be connected to a dog-lead, in order to let Mistress take the slaveboy for a walk.

If you tie the rope around the whole thing, you'll get an effect that'll increase an eventual erection as well.

This option is expandable, if you start in front of the ball sack, cross the ropes on the backside of the ballsack, and tighten it over the penis.
If you're applying this expansion, I'll recommend that you tighten it with some precaution.