I feel your fear. Knowing a loved one is facing a stroke counts as one of the most frightening, personal things anyone can undergo. I'm sorry it has to happen to anyone, and I can't predict how this will turn out, but I pray for his survival and recovery.
A word of hope if he pulls through, my father survived three strokes with his personality generally intact, as well as the ability to walk. It was around 6 years ago that the ordeal began. He was doing far better by the end, and I fully believe he'd still be around had he not clung to separate unhealthy habits.
Things will seem unimaginably awful at first, but they should stabilize once the right treatment is found for him. Offer him sympathy and conversation, even if he gets angry or vulgar - you can't judge a man who's been through something like this for acting out of frustration or misery.
Things may not go back to how they were, but be patient. Encourage better health habits, but don't force them. Don't wait for a miracle, but notice and savor progress. And in the meantime, if you're on good terms, keep it up like nothing changed. They will appreciate you putting in the effort to keep this event from dominating their life.