If he's 12 his tastes may change significantly over the next few years. I would use the fireworks theme in a way that it could be easily updated to something more grown up.

Framed photographs of fireworks?

Does he have a smallish wall that you can paint black and then paint fireworks on? Ooh, check these out:

Here is a link that might be useful: Fireworks wall decals

Sasha's Mommy idea reminded me of one of my favorite art projects from elementary school where you completely covered a sturdy piece of paper with all your favorite crayon colors. Once that's done you cover it over with black Tempra paint. Once the paint is totally dry you scratch off a drawing. It's called scratch art, picture linked below.
Though that idea isn't practical for a wall you could get a similar look by painting one wall in his room with black chalkboard paint and let him draw "fireworks on it using colored chalk.

Here is a link that might be useful:

How about doing a red, white and blue color scheme--in terms of paint, wall paper, bedding and WTs. Then add pictures and posters of fireworks.

What if you took some fireworks images, converted them to Black and White, and framed them at sizes of about 11x12? You could do a pattern of four images. That would be more flexible in terms of blending with whatever the color scheme is (or will be) and would give it a more grown up feel that may stand a longer test of time.

Thanks everyone for the great ideas!! I was thinking I needed to paint fireworks on the ceiling/walls, but since he is 12 and will probably change his mind in the next couple of years, I think I'll do pictures or murals that can be easily removed.

Anne, that's why I thought of chalkboard paint. Teen boy quite often love black in their bedrooms and with the black chalkboard paint on one wall (others could be anything but I'm thinking white) it would be easy enough to change his room as he grows without repainting. Just erase the drawn on fireworks and draw on what he's into next.