OP- I relate completely to your BF.
This is definitely conditioning possibly mixed with a weak immune system. But honestly, the conditioning part would make most sense- Unless, does your BF get sick all the time?
When I am sick, I liken all those symptoms to the dope sick I've felt of the past. The sweating and cold -[cause that really does happen when you have a fever, than break it- naturally or with ibuprofein of something]
This is totally normal, it just sucks having those flashback feelings. Psychosomatically- these feelings are real. Your psyche has a lot of effect on your physical wellness many a time.
The minute I get an runny nose that isn't allergies, I get a pit in my stomach feeling because that was the first sign of the beginning of dope withdrawal for me before all the worst stuff came on.
For me- There just is something though that completely totally separates dope sick from real sick, regardless if I feel so many similar symptoms due to past association of being sick. There's the extra body pain[indescribable- just IS.. mental pain, physical] that's unique to w/d...and the malaise.
And additionally, while being on dope, I never "got sick" from a legit reason. Bbecause the dope seems to cancel out the feelings of the immune system working hard. And since the only sick for a bunch of years that I was used to feeling was only dope sick... Once I was off the dope... any sick I felt would be likened to that.
ok, sorry - long! Hope I made sense!