Dear Claudia,
First of all, I am sorry to hear that your son got Tinnitus and Hyperacusis. Everybody makes mistakes (I played with fireworks too as a child) but it's not fair to get it at such a young age and it can be very unsettling for him.
I am not sure what is the best remedy, but I think what you should do is to first g
et a couple of good earplugs for him. I recommend custom mold earplugs meaning that they fit his ears perfectly and they are super easy to put in. Also, these plugs do not stand out too much, so he might not be too embarrassed being seen wearing them. I personally use Elacin earplugs, but I am sure other brands will do just as fine. You have special filters which can attenuate certain frequencies more than others, so he could for example still hear conversations well while other sounds would be attenuated more. This is simply to protect his hearing so the T and H will not get worse in the future.
Also, I think that he should not avoid getting out, playing with his friends etc. but I would recommend using earplugs in noisy surroundings. I got T when I was 23 (from playing in bands, going out in loud clubs etc) and I immediately got earplugs and was able to keep on playing in bands etc and going out until at least 35. I stopped doing that because I really don't want to damage my hearing any more. T can have a big impact on your social life, that is you don't wanna go out anymore, don't take a bus or train or airplane because it's too noisy etc. I think your son is too young and if possible he should enjoy life as much as he can, but he might need some help to avoid noisy situations.
Also, I think most of the people at this forum have had the experience of habituation. That is, you will get used to this sound although I know how hopeless and anxious and desperate one can get from hearing it all day long. So, it is very likely, although it may take a lot of time, that he will simply get used to it (if it stays the way it is). I personally fell on my head about a month ago, and my T. is also much louder now than it was before, but I think that over time I will get use to this new intensity as well, though I am still bothered by it now.
Also, one thing I can recommend is to watch his sleep. My personal experience is that if one doesn't sleep the T will get (subjectively) much worse. My strategy is always to get my sleep back to normal (I occasionally take some medicine, but its better without) and from there tackle the T. You can also buy a noise generator and put it in his room to mask the T (I use this one:
https://www.amazon.com/Sound-Fideli...ve-Technology/dp/B002SMJQT4?ref=ast_p_ep&th=1 but I guess others would be fine too, or you could start by using an app on the phone like Sleep Pillow).
In any case, your son is not alone, many people have this condition and learnt to cope with it, and for many the intensity of the T. also went down over time, so this will very likely also happen in his case.
In any case I wish you and him good luck with the situation.
Take care,
Rinz