Howdy. The lad is indeed wearing a hat akin that the Army regulations. Hats customarily have a brim, and cover the whole head. Caps don't have a a brim, or a partial one called a visor.
The army hat was indeed unpopular among many of the troops in active service in the field, at least in its regulation manner. Where worn it was often without the trimmings, and akin a common "slouch" hat, like this 14th Wisconsin drummer at Chattanooga...
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the hat:
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The forage cap...
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The light artillery cap...
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So that lad in the photograph is also wearing a uniform coat. Notice anything about it?