So he comes to me next year and I look at his 2020 return and notice the mistake, if he hasn't already told me about it.  Do I play Enforcer?  Do I tell him that I won't do his 2021 return until he coughs up some taxes he owes IRS?  What if the next new client brings in a 2018 return with an obvious error?  Do I demand that he file an amended return?  Is my obligation to the client, or to the government?

You made a mistake.  Tell him, and offer to correct it for no additional fee.  Explain how the CP-2000 program might not catch up with him for 18 months, if the tax involved is significant.  If you're not comfortable working with him next year, that's your choice.  Filing an amended return, is his choice.