Xanax is a benzo like valium, but shorter acting, and should calm him down I think. I'd ask the vet for side effects to look out for, because (NOT trying to make you more anxious) I think the benzo drugs can provoke an allergic reaction in a very few cats, so prob best to start him on a very small dose? Having said that, mine have taken it and valium when necessary with no ill effects except grogginess and increased appetite afterwards.
I think that the herbal alternatives like valerian and scullcap are probably not strong enough (?) if the fireworks are really near you. I'm so sorry, what a pain in the
this time of year is.
cuddles to Charlie xxx hope it pours with rain tonight.