Hi and welcome,
What you describe doesn't sound suggestive of MS and i too have never heard of Migraine adult or paediatric presenting like this either...
"His symptoms include the following: he will wake up in the morning with energy as a person who just woke up would, within a couple to three hours, he’ll lose his charge.He loses his full body strength, making him unable to walk, hold his head up, keep his eyes open and mouth movement (mimicking someone with MS)."
This isn't typical of a neurological condition like MS, whilst it's common for a person with MS to experience pseudo relapses, the return or worsening of their specific preexisting neurological issues with physical activity, increasing temperatures, infections etc what you are describing is global (all over) muscle weakness which is more typical of myesthenic disorders.
"The cardinal symptom of all myasthenic disorders is muscle weakness that is induced or worsened by exertion. This is referred to as fatigable weakness. In healthy people, physical activity causes a small decrease in the number of ACh quanta released from the nerve terminal that does not impair the safety margin of neuromuscular transmission, but it is incapacitating in myasthenic patients in whom the safety margin is already reduced.
In some patients with CMS, the weakness is confined to muscles supplied (innervated) by the cranial nerves causing double vison, droopy eyelids (eyelid ptosis), facial weakness, hypernasal or slurred speech, and swallowing difficulties. In other patients, the above symptoms are combined with weakness of the limb and torso muscles causing generalized myasthenia. In still others, the weakness is limited to the limb and torso muscles causing ‘limb-girdle myasthenia’.
The myasthenic disorders caused by defects in enzymes required for protein glycosylation can also be associated with development delay, seizures, intellectual disability, neuropathy, and metabolic abnormalities of different organs."
https://rarediseases.org/rare-diseases/congenital-myasthenic-syndromes/
I would recommend contacting paediatric Neuromuscular disorder diagnostic specialists eg Gillette Children's Speciality Healthcare, The Neuromuscular Disorders Program at Children's Hospital of Wisconsin etc and consider getting in contact with neuromuscular organisations eg American Association of Neuromuscular & Electrodiagnostic Medicine -AANEM, Muscular Dystrophy Association, Dyspraxia Foundation USA etc for advice and support..
As a non parental family member your unlikely to know all your nephew's testing and specific diagnostic evidence that he's had over the years, his test evidence and abnormal-normal neurological clinical signs etc are significant in what potential alternative neurological related diagnosis has merrit but if his parents haven't done so as of yet, i would suggest they consider genetic testing to rule out congenital neuromuscular disorders since this began so early in his development.
I would expect he already has a physio therapist, speech therapist etc and school related accommodations already in place but if there is concern in regards to changing of schools it is always recommended discussing the situation with the intended schools disability support coordinator prior to the exchange.
I hope that helps......JJ