Edit: I see you mentioned that you ruled out Pediatric stroke (I am not familiar with this,) so that seems to answer my question. hmm...
If the patient were elderly, just based on the symptoms you described (and I have no medical experience and thus cannot offer more than my own thoughts,) I would suspect a blood clot.
For example, my elderly grandmother was temporarily unable to talk well, or at all briefly if I remember right. Those symptoms gradually improved after she was taken to the ER. After doing some ultrasounds, the doctors found blood clots in both her legs. I believe they called the symptoms a Transient Ischemic Attack [TIA] (a mini-stroke.) They performed surgery to add a filter that was designed to break up and prevent future blood clots.
She seemed to improve relatively quickly once she was in the initial ER. I wonder if they gave her some initial medicine or something that helped. I am unsure. I think later they did maybe if anything. I believe it took a day or so for them to perform the surgery because they needed to run the tests to determine the cause.
She has other elderly health conditions as well. Thus, there could be more than one problem existing here. But that's my shot in the dark. Take it with a grain of salt of course.
I will pray for improved health for your son.