it would be extremely unusual for him to be sick after 4 days of a single drug overdose/exposure, what you describe seems like normal opioid withdrawal. Are you sure that was his first time using it? I hate to say it but if he is sick and unfunctional for 2-4 days of being sober it sounds an awful lot like withdrawal, addiction. That would only occur after prolonged daily use.
but we can only speculate, if he truly did overdose-- why no doctor? Opioid overdoses the person simply goes mostly unconscious and stops breathing/agonal breathing. If he is experiencing severe symptoms after an acute overdose 4 days later, and it was NOT withdrawal, I would blame that on more serious non-drug related conditions.
my heart tells me he may be doing more drugs than he tells you, but hey, I'm paranoid and don't want to make you think something that isn't true, I'm just going off what I read here, it could be something totally different
I highly suggest you keep an eye on him and/or go to the doctor again
people who OD on fentanyl do not feel bad 4 days later unless something is very wrong or some damage was done. Most junkies who OD and revived are right back at it within several hours and feel relatively normal.
pure speculation: but it's possible he is hiding a drug habit from you
(also, don't worry about triggers on this website, lol)