If there is a place that he likes to hang out, I would move some food there for him. That way he'll not move around as much. Also, you may want to move the water bottle so it is easier for him to reach as well. You can give him bits of food like apple that have a high water content. Not too much though. There really isn't much you can do for a gerbil with a broken leg. Most heal up just fine. Just keep him warm and in a quiet area.