Here's a new theory: Don's child was ...
Sun Green, and Michelle Pepe was her mother.
The letter was a completely unrelated red herring cooked up by Sherry.
I know the gravestone for Michelle reads "Beloved daughter and sister," but maybe Michelle had been an unmarried mother and the whole relationship had been embarrassing to her family, so they elected not to include her status as a mother on the gravestone.
Sun is about the right age (19), seems to take an unusual interest in Don, not a sexual interest, but a caring interest, the way a daughter would if she saw her father with a black eye.
Sun works and lives a very short distance from the cemetery where Michelle is buried, so they lived in the same general area.
It's obvious in the scene in the ceremony Michelle was the one out of Don's former loves he really did care for.
So my theory is Don went looking for his son, while he should have been looking for Sun.
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4) You ever seen Superman $#$# his pants? Case closed.