Unless he's one of the rare prodigies, I wouldn't suggest anything too complicated for him now at age 5. I was only able to show my 5 year old grandson the moves. His attention span was too short for anything else. Just stick with the basic's - value of the pieces, simple tactics (forks, pins, simple checkmates such as the mate-in-one puzzles you've been doing). You don't need the Polgar book yet. You can set up your own mate-in-one puzzles. Chess is first of all a game and should be fun. Let him enjoy it while he's young. Making it like work now could be counter-productive in the long run. Your approach sounds correct. Serious training programs are work, and my opinion is that they can wait till later.