I'm not sure how much of this might be useful so if it is, great. If not, please ignore.
You might want to do some exploring when you aren't having sex or as a kind of foreplay. Do the things you would do during sex and check in. How hard can you suck on them and should it be one or both? How hard can you squeeze them and from which direction? How far can you pull them and where does your hand need to be? Does it work to push them up or pull them down? If he's not worried that you're going to get carried away and accidentally hurt him or that he'll wreck "the moment" by tapping out, you may both get a better idea of how you can interact with his danglers. I'm not suggesting that this be about how much pain he can "take" but about what might hurt and what doesn't. Balls are like eggs, fragile and durable in very specific ways. Also, whether or not he knows your neutering fetish and how he feels about it might play in. I'm not sure if you've mentioned that. I hope you can (carefully and consentually) get this problem by the balls.