Spiritwalker: Messages from the Future - Henry Barnard Wesselman.

Hank Wesselman is a trained scientist, an experienced archaeologist. But all his training and experience barely prepare him for the realistic dreams he starts experiencing while his wife is pregnant with their first child. Recognizing these are no ordinary dreams, he expands his research on shamanism and realizes he is actually seeing into the future through Nainoa, perhaps a future reincarnation of himself, 5000 years in the future. Hank gets reaquainted with a childhood "imaginary" friend who leads him on a spiritual journey that changes his life.
ISBN 0553099760, Bantam Books, 1985.