Perhaps he needs to realise that there are other ways in which he can be the 'fun dad', but that this is off limits.
Not only is it illegal, and highly risky, but it is upsetting you. I don't understand why he isn't taking your feelings into consideration here?
There are lots of parenting (and other) things where DH thinks I worry too much/ am over the top, BUT he agrees to do them/ not do them because he knows how important they are to me.
I'd also be talking to your DH about the worst case scenario resulting from his actions.
As an example, my DH will let DD climb high play equipment, and encourages her to be a little bit fearless. He is there to catch her if she falls, and his reasoning is that he wants her to have confidence in her physical abilities. I'm a little more cautious. However, I understand that the worse case scenario is that she falls, and DH catches her, or he partially catches her, and she injures herself a bit on the way down.
While I don't want that to happen, I think it's important for her to gain confidence in doing these things, and I think DH is right. Therefore the reward is worth the risk (IMO).
In your circumstance, the reward is that your DH feels like the fun dad, and your DS enjoys it. The worst case scenario (I'm sorry to say) is that an accident happens and your DS dies. To my mind, the reward is not worth the risk. Not when there are so many other things that could achieve that same reward (e.g., your DH taking him to play centres), with very minimal risk. It's an unnecessary risk.