Mr Poonsak is the son of Pol Sub Lt Therdsak Saokaew, deputy inspector at Don Mueang police station in Bangkok.
Arhit died in hospital from shotgun wounds. His friend, Suwasin Pathummet, also a football player, suffered knife wounds.
They were attacked while on their way back to their dormitory after leaving a pub at Muang Ek about 4am.
The attackers had mistaken the victims for people they had an altercation with earlier, police said.
Mr Poonsak's alleged accomplice, Veera Sripong, 22, was arrested at his Sai Mai district home on Saturday and taken for a crime re-enactment the same day.
Following Mr Veera's arrest, pressure had been building on Mr Poonsak's father to turn his son in, which he did at a relative's home in Kamphaeng Phet province, Pol Col Amnart Chancharoen, deputy chief...
Police still are hunting for another suspect, Rachane Nisap, 17.