Emergency services were first called at around 5pm on Thursday, to Columbia Road.
A 14-year-old boy has been arrested, on suspicion of supplying a class A drug, in relation to the incident.
Authorities will conduct a post-mortem to confirm cause of death.
An investigation into the incident is ongoing.
Deputy Inspector Neil Third, from Dorset Police, said: “This is a very tragic incident with such a young life being lost and my thoughts go out to Mehmet’s family and friends.
“A post-mortem examination has not yet taken place, but we currently suspect that he died of a drugs overdose.
“Anyone with information that might help our investigation is urged to contact us.
“I would like to stress that it has not been confirmed yet why Mehmet died. However, I would like to use this opportunity to remind everyone of our advice.
“Anyone thinking of taking substances, please think again. You can never be sure what you are taking when you use illegal substances and the side-effects are unpredictable and potentially may have tragic consequences.”
Police are urging anyone with information relating to Mehmet’s death to come forward.
Deputy Third added: “We believe that Mehmet went out with his bike for a while between approximately 12.10pm and 1.45pm.
“The bike is described as an orange/red mountain bike with two different types of wheels.
“I am hoping that somebody saw him during that time and can help us establish where he went and whether he met up with anyone.”
More to follow…