Reddit mentions of Supportiback Wellness Therapy Acupressure Mat Set - Prick Free Acupuncture Mat and Acupuncture Pillow for Relaxation and Pain Relief - Massage Mat, Acupressure Cushion, Washable Cover and Travel Bag

I think the best form of feedback is silently looking him in the eyes as you torture him. You don't need to talk him through it and he doesn't need to make all that noise. Safewords are needed of course and involuntary grunts are unavoidable, but you will both be happier if he isn't required to communicate his pain level by constant groaning.

If you want to start slow, which is always good advice, pulling on his scrotum (the sack, not the balls) is pretty much risk-free. Basically it is a muscle that protects and temperature regulates the balls and stretching it is painful, yet safe.

Next, I would gently fondle his balls to get a feel for how they move, their shape, and how the are connected (the vas deferens) which is their most vulnerable part and which your want to avoid injury to.

For your first attempts at squeezing his balls you should take just one in the palm of your hand and wrap your fingers around it like an egg. Imagine it is a raw egg and you are trying to see how tight you can squeeze it without breaking it. You can even practice on real eggs first to see how tightly you can squeeze while applying even pressure. Generally speaking, you an squeeze as tight as you like this way without risk of injury.

Next, I would try slapping his balls. This is pretty safe assuming they are free to move around.

Now it's time to pay attention to his cock, which should be pretty hard by now if he is still enthusiastic. His head is he most sensitive area, so you can hold he shaft in one hand (pulling back the foreskin if needed) and slap the top of his cock sharply.

For his shaft, I would have a bag of clothespins handy and start a tight row down the underside of his penis. Then put rows down either side and one or two on top if there is room. See how many you can fit. Basically 4-5 rows with 6-8 pins in each row for the average size penis, or 24-40 clothespins. Put a few more on the head.

If you wish, you can finish him off by "cracking the nuts" basically doing the opposite of the gentle squeeze above. You want to slowly apply point pressure to his ball(s) until he can't take it any more. There are two ways to do this. One way is to squeeze both balls against each other. The other is to rest one ball in your fingers and press the joint of your thumb into the side of the ball. The second way is easier if you have small hands, but just be warned that it doesn't take much pressure to cause intense pain. Maybe hold one ball in each hand and switch to the other ball when he can't take it anymore. Be warned that his balls will be tender afterwards and he may not want to play for a day or two.

Honestly, if you are worried about safety and plan on making this a routine, I would suggest that a ball crusher device that can apply continuous pressure and can be adjusted gradually is much safer in the long run and is well worth the expense.

The style I recommend is https://www.amazon.com/PaddleDaddy-Acrylic-Ball-Crusher/dp/B00KO8WF10 Just make sure you don't over-tighten the cylinders. The should just hold the balls in place comfortably with no pinching. All the pain should come as the plate is tightened, squeezing the balls against the cylinders. The screws can be tightened every minute or two as the blood is squeezed out of his balls. Eventually they will be so tight that the pain won't go away, in which case you will probably need to distract him with pain elsewhere (pinching, biting, hard whipping) or maybe some pleasure selectively (teasingly) applied.