You can ask, but I dont think it'd be unreasonable for him to say no (what is there for him to say really? How could he sag much?).
Can ask, you are probably late at this point to shadow another and try to get a letter.
I expect > 2 days is more the norm for a letter, but I'm not sure. I was in a fortunate position and my DO letter came from a personal mentor, who was also my physician, and I have taken care of his patients.
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