What has been your experience US citizen at border with expired passport

My 21 year old son is here in Canada visiting me.

I drove 5 hours to pick him up. When I arrived, I found out that he did not get his enhanced driver's license, he got a real ID license. My son is 21 but on the autism spectrum and was relying on my dad to tell him correct information about what ID he needed, which was an obvious mistake.

My dad thought having "the star" was what matters... not understanding the difference between an enhanced license (My son resides in Michigan, where enhanced IDs are available and common) and a Real ID.

Given I hadn't seen my son in 2 years, I really wanted to bring him back with me for the week.

We showed up to the Canadian border, with his driver's license, birth certificate, and his expired passport in hand. After some basic questions we were allowed entry to Canada.

What should my son expect upon arriving back in the US?

He has his driver's license, his birth certificate, and a passport that expired in August 2023.

I know ultimately he will be granted entry, but how painful of a process is it likely to be to get him home?

He doesn't have enough time to be here to get a new passport at the consulate and he is expected back at work in Michigan the 5th of January.