The incident occurred between 3-4pm last Saturday at traffic lights on the A379 at Cockwood.
The suspect, a white male, in his late 60s or early 70s, was an estimated 5ft 10ins tall with fair hair.
He was wearing small lensed glasses, blue baggy jeans and a dark blue fleece.
A police spokesman said the man was riding an electric bike and had been seen along the roadside cycle path numerous times on previous weekends.
The 13-year-old lad rode off in the direction of Dawlish Warren and saw the man ride past him again.
Police would like to hear from anyone who was in the area at the time and who saw the male fitting the description.
Anyone with information is asked to email [email protected] or call 101 quoting CR/020309/20. Alternatively they can call Crimestoppers anonymously on 0800 555111.