Hi tw i have 2 suggestions - 1) let him fall over and hurt hiimself - perhaps a small knock will prevent him doing it again?
2) depending on the configuration of the stairs, either put something onto the 1st step so he cant climb onto it (anything will do) or put a sheet of cardboard round the bars of the gate, so there is nothing for him to hold on to.
i have a sheet of plywood sometimes at the bottom of my stairs to prevent the dogs going up, but i'm not sure this would help as my dogs are a bit stupid and don't have opposable thumbs, whereas your son does!