Yep we've done strap on play.
I love it
Hmm how about making him take it in front of my lover...
As you also say that you have had him service your lover orally, you could "spit roast" him by using the strap-on while he is sucking your lover. Then, if your lover is into the idea, switch places with him.
Another variation, if your lover is OK with fucking your cuckold's ass would be to give your cuckold a challenge. Position yourself seated or laying at the edge of the bed in front of him so that he can just reach your cunt with his mouth while he stands bending over. Have your lover sit in a chair directly behind your cuckold so that he can squat down onto your lover's cock. Adjust positions until your cuckold can fuck himself on your lovers cock while he eats your pussy, but in such a way that he has to constantly move up and down on your lover's cock while he tries to move forward enough to lick and suck your cunt.
The idea is for you and your lover to relax while your cuckold does all the work to get the two of you off. Set a time limit, lets say 15 minutes starting from the time he has found the optimal position.
If he can make you both cum at the same time, he gets a reward. Make it something he would really enjoy so he has an incentive.
If he makes you both cum in the allotted time, but not together, he gets the satisfaction of performing his duty well.
If he can only get one of you to orgasm, he may be given the choice of completing his task beyond the time limit or submit to a minor punishment determined by the one he failed to please. Continuing is a form of punishment.
If he totally fails at giving anyone an orgasm, he suffers whatever you and your lover deem appropriate for failure to please.
This is not an easy task as he must pay attention to how both you and you lover are reacting to his efforts to give you both pleasure, while concentrating on the actual licking, sucking, and using his ass muscles as he strokes your lover's dick with his rectum. All that while the muscles in his legs and back begin to weaken from the strain and cause him much discomfort. By the end of the time period he will have experienced much humiliation and will be exhausted and quite sore (in more than one way).
However, he will begin to discover if he is truly a slut or not and you will learn by the effort he puts forward to please as to just how devoted he is to you.
Success or failure isn't really the yardstick here as much as his desire to please and the effort he makes to that end, and you can always repeat this exercise occasionally to give him further opportunities to improve his performance while suffering the humiliation of being used as a mutual masturbation toy by you and your lover.