Just watch over him. Beyond telling him you care, take him food and make sure he actually eats something. If his clothes become dirty, see if he'll let you do some laundry. Avoid making him dependent on you, but drop by now and then to see how he's getting along, and do exactly that: see how he's getting along, and help where you see he needs it. Other than that, give him space and time. Everyone has to get through grief in his own way, and what he needs during the journey is both knowing he isn't alone, and a little help in the basics of life he otherwise might neglect. There isn't anything more you can do.