I love that, MissStress!
There are two ways to use the tunnel plug I bought to humiliate your sub. The first I already mentioned, and I list it again. Here's the link to the tunnel plug that I bought:
https://perfectfitbrand.com/tunnel-plug.html
(2.5 inch size medium, cheaper from Amazon)
1. It makes the sub feel so vulnerable and helpless when he or she can't control anal discharge. The cool drafts inside the rectum add to the sense of vulnerability. If you use a 1-inch diameter enema nozzle or colon tube, you can inject an enema through the tunnel plug with little leakage. Then you can cork up the tunnel plug and let your sub expel later if you want. After expulsion, you can humiliate your sub by letting him drip everywhere and down his legs. Outdoors might be better than indoors. Sounds like you've done some of this.
2. This plug also fits my mouth perfectly, and it might also fit your sub's mouth. At first it seemed too big, but I pushed hard, which hurt the corners of my mouth, and it popped in. It stopped hurting one it was in, but speech was impossible. I could only make unintelligible sounds and heavy breathing noises like Darth Vader. I could not move my teeth at all, or use my mouth or facial muscles to expel the thing. It was in so tightly that I had to pull it out with both hands.
So you can get two of these tunnel plugs, restrain your sub, and make him totally helpless, humiliated, and vulnerable both orally and anally. (You could also just get one for his mouth and use the aluminum one he made for his ass.) I don't know how big the aluminium one is, but if you plugged him at both ends with the plastic plug, he'd be helpless to fight off a 1-inch dildo up his ass or into his mouth.
Here's an idea. Partner with a male dom to give your restrained sub more than dildos or colon tubes in his mouth and ass. (Your male partner dom would use a condum of course.) Wouldn't it be fun to force your sub to give oral sex to another guy? (You'd use a safe sign of course, because safe words wouldn't work.)