DS is pretty good for 5, but not a prodigy or anything. We aren't sure how good because we aren't chess people at all. We certainly do not sit around coaching him and he can go months between games, but recently he's been into it. He loves to play, but also still tends to cry sometimes when he loses, though this is much better when playing strangers.
Here is a checklist of beginner's knowledge. If your son can at least do (1) (you can practice with him) he may be ready for a tournament.
(1) Can checkmate with king and queen vs. king from a random starting position.
(2) Same as (1) with rook instead of queen.
(3) Knows how to castle on either side of the board, and knows when castling is disallowed.
(4) Knows what stalemate is -- can construct a stalemate position with king and queen vs. king.
(5) Knows to start the game by moving the pawns in front of the king and queen and bringing out the knights and bishops.
(6) Knows the rank order of the value of the pieces.
(7) Knows what the four move checkmate is ("Scholar's mate") and can defend against it.
