I don't understand what the issue is. How would allowing him to go enable any sort of bad behavior on your husband's part?
Usually, we advise to do what's best for the kids. Sometimes that's tough, like when a betrayed spouse asks "Should I offer to take the kids for the weekend, even though it's not my weekend and I know my husband is going to be with OW?" or some such. But in the nerf gun thing case, I'm failing to see what the harm would be if you just allowed your husband to supervise your son so he could go?
