If there were a down vote button, I am sure I would get a lot of those at this point, but I might suggest just buying a reprint set...he could play with them, lose them, crease them, and flip them without any serious consequence. Your son is still very, very young.
I absolutely would recommend picking up a copy of The T206 Collection
http://www.amazon.com/The-T206-Colle.../dp/1931807949 or probably available directly from the author.
and reading about a couple of the players before bedtime each night.
He could find his favorites and then the two of you could, at a slightly later date, set out to purchase originals of the cards of players he finds most interesting.
I have been taking my son to shows since he was 6 (he's 21 now). As tempting as it is to just "do it for him" so far as buying him a lot of cards, the joy is in the personal journey, which is not to say you can't enjoy it together. It's fun to give them an allowance at shows and allow them to find what interests them most. And yes, I absolutely deviated from the allowance from time to time.