I think that might be a common U.S. M.1816/22 percussion conversion which does have a sideplate of that shape; unfortunately, the way he's holding it at the wrist makes it a little difficult to determine if it has that particular wedge-shaped butt or not:
I do not see a rear sight, which leads me to think that it is a musket rather than a rifle. Certainly the sideplate resembles the Model 1816 "standard" converted to percussion and the Model 1842, yes? Also, don't many Lorenz rifles have a cheek swell on that side of the butt stock? (reverse image).
You soldier is armed with a US M1822/28 Musket, commonly referred to by collectors as the Model 1816 type 3. It looks to be a National Armory "cone-in-barrel" percussion conversion.