You've been a good boy for me - not touching your cock and saving that big load of cum just for me.  I'm impressed and think that you deserve a reward!  Would you like that?  Wonderful!  But first, I'm going to have to lock your cock up.  Don't worry, you'll still get your reward.  

Next, I'm going to instruct you to finger fuck your assshole and milk your prostate.  That's right, open up that ass - I want you wide and ready for me to fuck with my strap on while I pound my own ass with my favourite anal probe.  Does that feels good?  Having you ass filled with my cock?  I can see your own cock swollen and throbbing in your cage and I'm not going to stop fucking that ass of your's until you cum through your chastity cage.  That's right, show my that you are a good slut boy and milk that prostate until your cage cock explodes!