At one point (1920 I think) there was an amendment which made alcohol illegal. It went on for 13 years and then was removed.
So there is a legal framework to add/remove amendments from the US Constitution, they are amendments after all, they were not there originally (I think 3 years after).
It was removed, but also (and maybe because) it was nearly impossible to effectively enforce. Gun restrictions would very much experience the same problems. There are millions of guns and gun owners in the US and many of those people are firm believers in their right to own a gun. They'd likely bury them in the yard or, as the old joke goes, "lose" them in a boating accident, before turning them over to authorities.
I personally would _never_ willingly turn mine in--they're for my and my family's protection and have saved my life before. I am certain many gun owners feels this way as well. Trying to enforce such a rule would be difficult, if not impossible.
I didn't really comment on if it was enforceable, just that in a logical manner there is a path for amendments to be removed, added or changed.
Doing minimal research on prohibition since I had no context into what triggered it: "Prohibition also united progressives and revivalists. The temperance movement had popularized the belief that alcohol was the major cause of most personal and social problems and prohibition was seen as the solution to the nation's poverty, crime, violence, and other ills." [0]
You could replace alcohol with firearms and the part about removing them being "solution to the nation's poverty, crime, violence, and other ills". I bet there are a lot of people that would believe that prohibiting firearms would help solve those issues.
Just because laws are not easy to enforce doesn't mean they can't happen. There are still counties in some states that are 'dry'.
I think he was alluding to the fact that although there is a legal way to make guns illegal, it won't actually get rid of guns, in much the same way making alcohol illegal didn't prevent people from drinking.
So there is a legal framework to add/remove amendments from the US Constitution, they are amendments after all, they were not there originally (I think 3 years after).