What I'd do, in this order:
Keep DS home a couple days, and encourage him to hang in there through Christmas. He'll have extra time off for Thanksgiving. He'll have about 3 weeks in December before Christmas break, and he'll have fun things like Christmas parties & usually a program during those 3 weeks.
Call the psychologist and try to speed things up. You want to get action before Christmas! Our school requires 90 days advance request before accelerating a child.
If you keep DS home a few days, then send him back while you wait on the psychologist, I would not tell the school anything other than DS didn't feel well. If the dr has a plan, then you can tell the school why DS didn't feel well.