Tintypes were first invented in 1852 by Adolpe Alexander Martin, but were patented in 1856 by Hamilton Smith of Ohio. A rival, Victor Griswold, patented a similar product which he called a ferrotype and later patented with the name tintype. Tintypes were actually thin sheets of iron on which multiple copies of the photo were printed and then cut.*
Another aid in dating this photo was that this was actually encased in a gold metal frame (see below) which I tried to scan, but the photo came out blurry. I removed the tintype so the boy's photo could be clearer. These metal frames were used when tintypes were first introduced.
*Source: Allen County Public Library, A Tintype by Any Other Name