Pull his arm sideways while sinking weight, stepping back and away from his incoming feet. In this case though the kotegaeshi ukemi will not save him as he will be having lateral instability added to the vertical instability. Basically, he will land shoulder, head or face first on the floor and be dragged a bit. Not a nice prospect if one is thinking about protecting one's aggressor, but will work if the foot is coming too close.
I don't recommend one try to practice this with a partner who you would like to show up the next day to train though. On the other hand it is best to practice technique with correct positioning and distancing to avoid the foot becoming a threat to begin with.
Just my 2 cents.
LC