Another thing to look up is Postural Orthostatic Tachycardia Syndrome (POTS). This is another autonomic neuropathy or dysautonomia that sometimes occurs with GBS or separate from it. Ericainorlando that is currently posting on the site had this. She might be able to give you some good advice about ways that your son could manage some of the symptoms of a messed up autonomic nervous system.
WithHope

My cousin’s granddaughter, who he has raised all of her life, is 16 years old now & has had a very severe case of POTS for over two years now. She is basically homebound & cannot even attend school. I live in MN & they live in the state of Virginia, so I have never met her. But if you would like to talk to someone who knows a lot about this illness, I know he would talk to you. Just shoot me an email & I could give you is name & phone number…
Pam