Depends what he's into and how much you want to spend. I usually go up to about £20 for godchildren and a bit more for a family members. if it's your own child then I'm guessing you would spend more?
Lego
Things for whatever activities they do - i've had requests for sports kit, stuff for Scouts, swimming towels, parkrun wristbands, club hoodies etc.
Books or book tokens - he's probably at an age where having his own "money" to spend will be a novelty. A sweeping generalisation but boys tend to enjoy non-fiction books.
Clothes / PJs / Onesies
Theatre tickets or tickets for an attraction - may be a bit harder at the moment as you have to pre-book specific dates. Or a "voucher" for a day out together.