Look at his hair - he has a cowlick or a natural curl on his right (left side in photo). It's in the photos of Lee and it's apparent in this photo. He doesn't look like Whitman or Bell - look at the shapes of their heads. Lee wore his beard the same way most the time - there are too many points of similarity (to take the term from fingerprinting) between Lee and the photo for it to be any of the other suggestions.

Kendall's Photo and Robert E. Lee (enlarge that one and compare the brows, the nose, the beard and hair). Kendall's photo has whiter hair, so I'd guess that is later. Lee died in 1870.

SRS