I'm not sure it needs to be weaned really. How about omitting it and offering lunch when he wakes up? Offer water in a cup with lunch. Or if you feel you need to wean it for whatever reason, offer some ounces of formula in a cup, and water in another. You can also offer a small snack and water prior to his nap.
If it were me, I'd omit the bottle and offer water in a cup with lunch. I'd offer a bit more at lunch to be sure he isn't hungry from omitting the formula.
Best of luck