The emergency services were called just after 1.30pm following the collision which involved a gold Peugeot 206, registered to a woman from Wimborne, and a male pedestrian.
The incident took place near the junction to Darby's Lane.
There are not believed to be any life threatening injuries, but PC Lee Savage, speaking to the Daily Echo on the scene, said the victim had received a serious head injury.
The car was being driven by a local man who was uninjured. Police say he is helping officers with their enquiries.
PC Savage added: “We are aware that there were other vehicles at the scene at the time of the collision.
“We would appeal for the drivers of those vehicles to get in touch with PC 287 Gatfield of Poole Traffic Police on 101.”
Samantha Stockley, 18, who lives close to the scene of the collision, said she went outside to help the victim.
She said: “We heard the sound of a massive crash upstairs in our house and when I went outside I saw the boy lying on the floor.
“Somebody gave me a pink cardigan to wrap up and put under the boy’s head. When he came to, he was just asking for his dad. It was horrible to see.
“I think he had been looking at the bus timetables just before it happened. As soon as the ambulance got here, we just moved out of the way so that they could help him.”