Advice: 21 year old ADHD son goes catonic; Rigid body /smirking face.

Has anyone else seen this?  What does it mean when son goes completely catatonic?  He stands rigid.  You could actually knock him over.  No eye contact.  Smirk on face, with expression held like that.  This could go on for an hour.  Nothing reaches him--not touching him, speaking to him.  No clear sense of what triggers it.  First occurred maybe 2 years ago in the midst of ADHD battles and pleas over deadlines, etc.  Most recently though, it can happen when relatively neutral questions arise--like deciding where to go for dinner when his father & I visited him for college parents weekend.

Has anyone here had experience with an ADHD diagnosis masking a bigger issue that needs attention.  Co-morbidity?  Does this sound like he might be bipolar or have some autism spectrum issues?

He'll be home soon after apparently flunking out of freshman year at college.  He's resisted meds for 2 years, started taking them this month.  Is out of meds and needs to see a prescribing physician and the time seems right for a new evaluation.

Is it possible that ADHD is the tip of his iceberg?  (He repeated his junior year of HS, and then his senior year, at a school for ADHD kids.  Despite daily focus groups and specialized curriculum there, he seems to be getting progressively worse.)

I feel like we will get nowhere unless we know the right questions to ask the doctor (based on years of painful experiences).