Hi there. DLCs are licensed by the platform to the account they are purchased with and are not transferrable. You can share your games and DLCs with sub accounts on your console, but for your son to have ownership of any DLCs each needs to be purchased with his own account. Season passes are applied to the first account to log into D2 on the platform they are purchased on and go wherever the characters of that account go. Please note that any gear your son has gained by playing any DLC not rebought on his account he will still be able to use without restrictions. He will only lose access to activities and quests exclusive to the relevant DLCs not owned on the account until they are purchased.