here in my location children aged 9 do not use terms like I feel blank to describe how thy feel....here the word "blank" is a filler word for swearing, but knowing they shouldnt use the swear word, its also a word 9 year olds encounter in school (ie please fill in the blank to this math problem)

reason I tell you this is because when you asked him what he meant he could not tell you. here in my location when a child tells someone they feel something then when asked what they mean and they say I dont know thats a sign that someone has coached them what to say but did not explain what was meant by what they are saying, or a child has over heard someone else say something and they copy with out knowing exactly what they are saying.

Im wondering if during the times you have confronted him, he picked up on how you ....want....him to say something is wrong because you keep asking him even after he says he's ok, so now he's maybe saying what he thinks you want to hear but know understanding what he really is saying.

here in my location usually when a parent feels something is wrong with their child the first thing they do is talk with the doctors. I see by your post the doctors are trying to diagnose the problem. my suggestion is the best thing you can do for your son\to help your son is to keep working with the doctors they will be able to tell you how to best approach your child and this problem and what needs to be done to narrow down and diagnose the problem.