"so my Oncologist's college student son, if not claimed by the parent, can get a stimulus?"

No.

The wording is "if you can be claimed as a dependent" and not "Won't Be Claimed." In other words, a dependent, even if filing separately, does not qualify for Stimulus on their own.

Why do you not believe how the software is handling it?

"these kids are not being claimed by the parents, so they think that that makes them, 'not a dependent' and eligible for the stimulus"

A professional preparer is signing that you are the responsible party, though.

"not being claimed" is not the same as "can be claimed."