Cutie mark liable to change, but for now it’s a cumulonimbus cloud.
He’s a very bulky stallion, as pegasi go, and not a very fast flier. But creating turbulent and massive thunderstorms, it isn’t speed that’s needed, it’s strength and balance, which he has.
In my head, the way he creates the long, rolling rumbles of thunder (hence his name) is from the way he rakes his feathers along the clouds as he glides through. That’s liable to change as well, as I still need to, y’know, write him into the story.