Best thing to do is to pray for the repose of JFK's soul. If he's in Heaven, he doesn't need it, but no prayer is ever wasted. If he's in Purgatory, he'll be grateful for the prayers. If you're worried that JFK might not have repented, you are always welcome to pray a Chaplet of Divine Mercy and ask Jesus Our Lord and Savior to apply the prayer to the moment of JFK's death, for a miracle of grace of final repentance as necessary.