The teenager netted 10 times in 15 starts for the club, and leaves Huish Park with Slade's best wishes.
"I am delighted for Best that he got into double figures before he left. We shall miss him," said Slade.
"Bestie has been a tower of strength, not just with his goals but with his work ethic on behalf of the club. I hope one day we get the opportunity to bring him back."