I'm sure apart of him will still hear and comprehend your words. Don't be afraid to talk with him directly. You'd be amazed at how well your intentions will still come through, regardless if your words aren't 100% understood. Language and comprehension does not exceed the power of love.
Even coma patients when they wake up, have told their friends and family that they heard what they were saying to them. Even when they were not able to understand word for word, the knowledge that they were there with them was comforting.
I think you would be better off communicating to him in person and being there with him, which is a 100% sure way to show him your support, than frantically trying to learn lucid dreaming or astral projection in hopes that you'll be able to connect with him.
I wish your father good health and a speedy recovery. Blessings to you and yours.