You can click on 'move reminder' next to the game on your My Games page. Will send an e-mail to them, although whether that will cause them to move quickly is another matter. Although some people (including myself) prefer not to receive these.
Also if he is within the timeout/timebank controls there isn't much you can do anyway. Breaking those and you can then claim a win, sticking within them and you have agreed to that amount of time per move.