Some other ideas: try playing some Mozart quietly in the background. Play with him (da bird) if he'll do this. I find singing to cats soothes them. Also, don't get stressed yourself, he'll just copy your reaction to the fireworks. If he'll eat, give him a large, tasty meal: go to the butcher and buy him something special. After eating, most cats have the need to sleep. I'd also go along with the aromatherapy - lavender would be great. If you have some REAL catnip - at this time of year it'll be dried - sprinkle it around him, then he'll be too involved with that too worry about the noise. Cats react differently to this; some don't react at all.
I've been really lucky with Fred and Victoria, who were both feral kittens living at the back of some shops, during last year's firework week. They've been very laid back. Victoria was a bit nervous at first. I told her "it's just some silly old noise!" and that she was safe with me. I also imitated the noise in a gentle way, to try to convey to her that there was nothing to be scared of. She's been fine.
I hope Charlie is okay over the coming weekend.