any ideas of what i can try?

have you tried sitting him down and telling him how you feel?tried telling him that u know it was hard losing someone but your now afraid of losing your only dad,tell him the effects it would have on you if you lost him. tell him how much you care and need him.and that sometimes things happen to test us, to make us move up a step in life, and not in a negative way.also tell him that your willing to wait, and you'll be his crying shoulder.and its not always easy to tell a "man" that he can cry on your shoulder, but deep down inside they want to just break down,but they're never willing to admit it.If that dosent work,you said that he's always with his sister right? well tell his sister how you feel, and that the life style he's living is not healthy,tell her to talk to him,attack the person he is most close with right now with your emotions and tell them to help you help him.