Lee Bradbury captured Williamson from the Glenn Hoddle Academy earlier this year, but has seen the young striker sign for Port Vale.
Speaking in the Bournemouth Evening Echo, Bradders is quoted as saying: "It is a shame because he is a good player and I think he has a lot of potential. But I couldn't guarantee him first-team football with the quality of strikers we’ve got in the squad. I couldn’t promise that he’d be playing every week and I think he will be doing that at Port Vale, so that was an attraction for him".
Meanwhile, Eddie Mitchell has confirmed that AFC Bournemouth have rejected a bid from an un-named Championship club, believed to be Watford, for Rhoys Wiggins.