That is one rumour - but there is a much better modern one that says he wasn't murdered, was used as a hostage until his captor (Sir Roger Mortimer, perhaps the most interesting man in English history) was executed by Edward III, then went to Italy to become a monk and ended up meeting his son later on. It sounds mad but there is contemporary evidence for it:
https://en.wikipedia.org/wiki/Fieschi_Letter
Ian Mortimer's "The Perfect King" contains the best account of the survival theory, and is a fantastic book in its own right.