Charlie Caston had been on an agreed visit with his birth mother, Alexandra Konstacka, but was not returned.
Ms Konstacka lives in the Warddykes area of Arbroath. She may be driving a grey Seat Leon car, with the registration number SP04 WRX.
Charlie is described as having short light brown hair and brown eyes.
Police have appealed for information on a five-year-old boy who has been reported missing by a family member.