That's Don Glut at one of the comics fandom events in the early sixties. The same picture is in Bill Schelly's book "Golden Age of Comics Fandom".
The boy in costume is identified as "Dave Armstrong at an early New York Comicon." The photo was taken by Larry Ivie.
Glut says it's him on his documentary "I was a Teenage Movie Maker"; he did have a Cap costume like that in one of his home movies. Whoever it is, it's a great costume to have been made by a kid.
I'll have to dig up my copy of Schelly's book and re-read the caption.
It is Dave Armstrong. Dave was (and is) the last acting president of the American Association of Comic Collectors (AACC) and is still actively collecting. He is also accumulating tons of film footage of interviews with creators from the golden age.
Thanks for providing a direct attribution, Richard. Speaking of active collectors, I just found out that Earl Blair (Capt. Bijou) whom I mentioned as a source for your collector history lecture awhile back, has moved back to Houston and sells memorabilia on ebay! I just picked up a Henry Brandon signed Drums of Fu Manchu lobby card from him within the past couple of weeks. Weird coincidence, huh?