Hi Indie, this is usual play for most cats, it is basically what they would do to prey once caught, and there really isnt a way to discourage it as it is instinctive behaviour.
My advice would be figure out the spot that gets him going with the leg kicking and grabbing and try and avoid this area or type of play.
Well done for rescueing him, i volunteer at a sanctuary as part of my animal management course and so many are desperate for homes.
He will really come into his own over the nxt couple of weeks, just remember to keep him in for a min of 2 weeks so he knows you are home.