Philip found Nathanael, and said to him, "We have found him, of whom Moses in the law, and the prophets, wrote: Jesus of Nazareth, the son of Joseph."
All Jewish names beginning with "Bar-" are patronymic, indicating parentage, such names including: Bartimaeus, Barabbas, Bar-jesus, Barnabas, and Bar-Jonah, the latter being the surname given Peter by Christ himself  (Matthew 16:17).  If, then, Nathaniel was the son of Tolmai, it would be no strange thing at all if the synoptics referred to him as Bartholomew. John, in going back to the very beginning of things, would naturally have used the name Nathaniel.  This variation is no evidence at all against the apostleship of Nathaniel.  As Ryle observed: