This page of the guide explains whether Kratos' son, Atreus, is playable in God of War Ragnarok.
WARNING! This page contains spoilers. If you wish to play God of War Ragnarok with as little information as possible, read at your own risk.
Playing as Atreus
Atreus is indeed playable in God of War Ragnarok - you'll control him for the first time during the third main story quest, Old Friends. Atreus is a fully playable character with melee and ranged combat capabilities, unique skills, equippable gear, items to collect and companions to command.
GoW Ragnarok doesn't let you freely switch characters to play as Atreus. Kratos' son is only playable at specific moments during the main storyline.