A very interesting set of symptoms you have elaborated beautifully indeed.
Infections will be the first on the list with several viruses capable of causing fever, mylagia, hypotension
. Adenoviruses will be one example. However, low blood pressure is a sign of sepsis that can be secondary to viruses or bacteria. As he is getting better (and I presume without any specific therapy) a viral infection is most likely.
Low cortisol levels require excluding if these symptoms (dos not have to be all of the above) recur.
I also presume there was no skin rash
at the time which would be consistent with an infection.
(acute) does indeed present with low blood pressure, but also with vomiting
(usually blood), jaundice
(yellowing of skin), dark stools (reflecting GI bleed) so without these symptoms it is unlikely your son had acute Cu poisoning.
I hope this was useful.