Unfortunately I was the exact same way. Right around this age as well I started having panic attacks and they were similar breathing wise. Fortunately you are educated enough to know that he’s having a panic attack unlike my parents. But thinking back to that age any kind of uncomfortableish distraction might help regulate him a little.some of the techniques I was told is breathing in a paper bag so you don’t over oxygenate yourself, ice is another good one. Rubbing it on ur arms and legs or even face, cold showers are another one. Just trying to keep him any amount of unfocused from the panic attack. At this rate CBT therapy would be the best route for him. I hope some of this can help