Final answer:
The best course of action is to check the patient's blood glucose level immediately and prepare for intubation, administer dextrose if hypoglycemia is present, provide respiratory support, and transport to the hospital.
Step-by-step explanation:
In this scenario, the patient is showing signs of life-threatening opioid overdose, suggested by the unconsciousness, hypoventilation, bradycardia, and hypotension. The appropriate next step after unsuccessful administration of naloxone would be to check his blood glucose level as hypoglycemia could mimic opioid overdose and require a different treatment. Concurrently, preparation for intubation should be made by your partner. If hypoglycemia is present, administering 25g of 50% dextrose could be lifesaving. Additionally, continued respiratory support with oxygen and transport to the hospital is critical.