Word's Champion Guide to Chess by Polgar is perfect for him. Its puzzles are suitable for him.
https://www.amazon.com/World-Champions-Guide-Chess-Step/dp/1941270328/ref=pd_sbs_14_2?_encoding=UTF8&pd_rd_i=1941270328&pd_rd_r=BN9E0Y7KMXQZAA1TD9TK&pd_rd_w=pahAV&pd_rd_wg=1FvDE&psc=1&refRID=BN9E0Y7KMXQZAA1TD9TK
Tactics Time is also good.
https://www.amazon.com/gp/product/B009TBYA7U/ref=series_rw_dp_sw
My 6-year old is slowly getting obsessed with chess. The good thing is we live in NYC so there are many many options, from Washington Square Park, to lots of programs, coaches and tournaments. I've been exploring options for the past few weeks and am bewildered. Any tips? I would like to find a mix of online (chesskid app? chess.com?), regular books/puzzles from the bookstore or Amazon, and perhaps sign up for a program like ChessNYC.