If he's beyond dust bathing or preening (as critters tend to lapse on with age), making sure he's always near food and water is about as much as you can do to keep him alive. Keeping the temp regulated (i.e. bring him inside if he lives outside) and making sure the environment is quiet and clean should reduce the environmental stress.
Have you had a pet die before? If so, you know how hard it can hit. If not, you might start thinking about what you want to do with his body after, if you want to say some words for him when he goes, ways you want to honor and remember his life with you.
I'm sorry your quail is so old, but I'm also really glad he made it that long.
