Just walk away and completly ignore Jasper when he starts biting, and reward for nice play and calm behaviour.

Have you spoken with your Grandfather and explained that Jasper is not allowed to bite? And maybe suggest that your Grandfather plays a tug game with Jasper instead of the biting games.

Oh also some bitter apples (Spray) may help, you can spray a bit on your hands and the bitter taste should deter him from biting.

Goodluck and keep us posted.