I know in PA, a person could buy a shotgun with just their driver's license. In states like TX and NV you were able to purchase firearms too.
A former co-worker had lived in TX and was able to carry a pistol on him without a carrier's permit.
NJ has some of the strictest firearms laws, and I think NY,too. I would think over the years with all the stuff that has taken place almost all states would make it harder to purchase firearms. Laws vary state, even fireworks, one can cross over to PA,MD,DE, etc. and buy fireworks where as other neighboring states you can't
If there is any way you can inform the authorities that this man is a possible threat, and by owning a firearm(s) can endanger those around him, including himself.
You can call from a busy pay phone and not disclose who you are. They may give you crap, saying if you do not disclose your identity they can't persue any investigation,no probable cause,etc.
But do not doubt once the bug is in their ear, they will keep this as a to be watched.
I've been told by several police that due to rise in terrorism and gang related crimes, they are more on top of stuff. For all of our sakes I hope this is true.
To act before an incident occurs is a safe call, before it is too late.
Good luck with this and stay safe.

DE