Sounds like nerve pain to me. Why, if its not too too nosey is he in a chair. Has he had an accident, or is it something he was born with or developed. If he has a pinched nerve its going to be very painful sitting on it. I know because my daughter said to me the other day, why was I sitting in bed reading my book, bent only at the neck, and that was because I could not, and can not take any pressure for any length of time, on my rear end due to spinal stenosis, and sciatica. Only I am not doing my neck any favours either. Your son might find if he can spare some time to lie flat or change his position it might help. Sorry I have not been very helpful. Why does he not get good care?
Hope you yoursel are as well as you can be.
with hugs.
M.x