Joe

Joe was a fair man, with curls of flaxen hair on each side of his smooth face, and with eyes of such a very undecided blue that they seemed to have somehow got mixed with their own whites (8).

He was a mild, good-natured, sweet-tempered, easy going, foolish, dear fellow- a sort of Hercules in strength, and also in weakness (8).

I always treated [Joe] as a larger species of child, and as no more than my equal (9).

I think Daniel Radcliffe will be an almost perfect fit for this role.  Although he maybe be on the younger side to play Joe, he does have undecided blue eyes ("his mother's eyes" apparently...), he can also play characters who are easy-going and foolish in a way.