I'm CFI and one of my student haven't flown after he got his private pilot license in 1990's and he wanted to fly again. The thing is his medical is expired already and he doesn't fit medical standards anymore so we're aiming for sport pilot privilege(He isn't qualify for BasicMed also he got his medical before 2006). Can he fly as a sport pilot with his private pilot license with driver's license?
If his last medical expired valid before July 2006, it’s now like he has never had one at all. (A denial or revocation would be different.)
However, if he does not believe he could pass a medical exam today, I would point out that FARs 61.53(b), 61.23(c)(2)(iv) and 61.303(b)(4) still prohibit sport pilots from flying if they are aware of any condition that would make them unfit to fly.
This rule is widely ignored in practice, but it is your responsibility as a CFI to point it out—and it would be wise to have proof that you did so in the event he ignores you and then has a health-related accident.