The hereditary prince is still a prince. His style of HRH and title Prince of Serbia (Yugoslavia) does not change unless he were to give it up, which he has not done.
I do not believe that the prince is in the states anymore, the last it was written on here was that Petar lived in London with Prince Philip. The youngest son Alexander is the only child of the three sons who is in Serbia right now.
Petar has occasionally shown up in royal events and continues to do so. He was last seen at the recent funeral of his Godfather at Oplenac.