Having raised a half dozen kids, and knowing just how frustrated a parent can get doing so, I would be inclined to approach with a smile and attempt to strike up a friendly gun related chat session, making friends so too speak. And then using good communication skills, I would try to inject myself into their recreational session by simply asking if I could let Jr. try out my Marlin 60, or what ever might help alleviate some of the issue. And then ask him if he wants to try on a set of my ear muffs to see how well they control the loud report. And I would try to keep on track with the whole recreational experience by getting Dad's attention with some firearm talk, what we like to shoot, and keeping comments in the mix like, " It's so wonderful to see a Dad and Son spending quality time together shooting their guns" , " And it brings back so many great memories I have of doing, what your doing with your Boy". Maybe ask Dad if he wants to try one of my guns too. There are ways to approach situations like this without causing tempers to flare up. The Dad might even appreciate the help of someone else, because it allows him some time to focus on his day at the range with a bit less time being demanded of him by his Son. I remember those days with my kids. Honestly, I would not get very much time to myself when taking them fishing or shooting with me, and I did get frustrated at times because I didn't get my day of recreation as I had hoped to. And who knows, a friendly interaction may lead to making a new friend too.
GS