First DO contact the Embassy, Surely by now he will have been taken to BKK for autopsy and the family contacted by the Embassy, they should liaise between you and the family as to what they want you to do. They can send a letter giving you permission to organize things. At this stage you should have a direct e-mail or phone number from embassy. They can recommend funeral directors. If you don't use them you'll be running around from pillar to post. hospital-police-police at hospital, body sent for autopsy, to BKK police forensic, to embassy, death cert , take death cert. to the Amphur office where he died/lived, register his death, back to BKK, release body, arrange for coffin, and transport to wherever they/you want the funeral. arrange funeral.
To save your sanity use one.
I can thoroughly recommend Asiaone.
Authorization Form - English.pdf