Angel Baby by Charles J. Hemphill

Tom Masterson becomes his future parents’ matchmaker before he’s even born. When a glitch in the celestial baby delivery system is discovered, Tom learns he will be the first of a generation of babies not to be born. To rectify the problem, Tom is sent to earth as an adult during the chaos of World War II with only two months to find his future parents, have them fall in love and marry so Tom can be born and the system correct itself.