But over-21 children of USC have to be in the US legally* to adjust status.
If his parent became a USC before he turned 21, he could adjust status if he entered the US legally (and otherwise didn't have any restrictions against adjustment, as I mentioned above). But he's 20 and his parent obviously can't become a USC before he's 21, so he won't be able to adjust status from his parent's petition, and can't adjust status through employer sponsorship either. He will have to leave the US and wait out the 10-year ban, unless he marries a US citizen. You don't have to wait for the 10-year ban to expire to file for him; he can wait out the 10 years simultaneously while waiting the 11-12 years for your petition to become ripe for a consular interview.
*some people in that situation can adjust status despite being here illegally, if they were in the US and were grandfathered in by having the relevant paperwork filed before the April 2001 deadline for 245(i). However you have given no indication that your relatives would qualify for that.
Based on reading: (uscis.gov /files/ pressrelease/ Section245ProvisionLIFEAct_032301.pdf) I can't see how this would have applied to him, of course I am not really sure I understand it completely but it doesn't seem so.
Anyways, just so I am clear what you are saying is that he would def have to leave and then wait 10 years outside the US to be able to come back in. Even if his parents becomes a USC sometime in the future? Basically there is no way for him to stay here without leaving for 10 years - right?