I lost my dad earlier this year and I was extremely close to him... in all honesty, I amazed myself at how strong I was for him. I unconsciously put all my thoughts and feelings aside and just gave him the love, support and attention he needed from me. You will be fine, just give him want he needs from you. If he is making a decision that may not be of what you wold do or want him to do, remember he is telling you because he loves you.... respect what is happening and what he chooses and just love and support him through this difficult time.
Hugs, positive thoughts and prayers to you and your father