After 40 years, if it was a felony back then and he has has a clean record since then, he should apply for a pardon. That would clear the recrd where an expugement would not do the job. In today’s political climate, he might get one for simple possession.
The article states that it was a misdemeanor.
I don't think that will work. I seem to recall a judge ruling that a even a dismissal after deferred adjudication still counts against you when it comes to buying a gun.