First, if you haven't already, install handles on the rollbar. I would assume you would want one for each seat, but its not absolutely necessary. That will give him something to grab onto to help pull himself up to a standing position on the seat. There are a few vendors that supply decent looking handles and the associated hardware that might chime in here.
Secondly, when he is ready to exit, try standing up yourself in the pilots seat, and grabbing on to his left hand to further assist with getting him into a standing position on his seat. With his right hand on the roll bar handle and his left hand in your right hand, that should get him over the hump. The rest I'll leave up to you, but an extra hand getting him off the wing might be good.
good luck