"Why does this man get left out?"

In general, I would point out his living situation hasn't been significantly impacted. He didn't lose a job, he didn't have children in a daycare that closed, he isn't having to homeschool them, he isn't at risk of losing his health coverage. I could go on. The problem is when the phrasing is "stimulus" or "recovery" or whatever someone believes needs to apply to them, specifically, even if it doesn't.

There is nothing in the 2020 regulation for this: "Is there a way for my client to receive stimulus payments, or alternatively, have his parents be able to claim a dependent stimulus payment for him because he is 100% disabled, in spite of being over 17?"

Not everything applies to everybody. I am sure we all can think of something that excludes each of us. Example: "President Biden's stimulus package temporarily increases the child tax credit to $3,000 or $3,600 per child for most families. Half of it will be paid in advance later this year."

Is your client going to continue to whine when all these people with kids get money, and he won't get money? There's still time to have a kid in 2021; he should hurry.