He can keep the license and firearm. I think he avoids Trump at all opportunities. Jace, you don't think the Secret Service has a file on Deniro? If they feel he is a major threat, they would take steps in legally having his firearm(s) and license taken away.
I don't think he is a serious threat to the president, but his constant rings and past drug use should disqualify him form owning a firearm in New York. It is almost impossible to get a carry permit here, yet he has one for over 30 years. People with carry permits in this state have lost them for less than what he has said. Johhny Depp also joked about shooting the president, yet he has gun permits. So do Whoopie Goldberg and Bill Mahar. Liberals want guns taken away from people, except when it comes to themselves.
De Niro was quested in the death of John Belushi back in the 1980's, and in a book by former baseball player Lenny Dykstra he details how De Niro and him went on drug binges together, with De Niro at one point chasing him to ask for some drugs when his own stash ran out.
Any person acting like De Niro loses their gun permit, sometimes permanently. Threatening the president was a crime until 2 years ago, we need to make it a crime again.