Can a TN visa holder file 485?

My hu*****and recently got Canadian citizenship and TN visa. I filed my 485 last August. He didn't file his 485 with me at that time since he cannot leave his job in Canada. Now that he got citizenship, and a job in US. When my priority date is current, Can he file his 485 as TN visa holder? Must he change his visa to H4 first, then file 485? If H4 is a must, he has to quit his job :-(