Hope he is OK today.
I also think its best to see your GP to exclude any physical pb. Don't be shy and give the GP all the suggestions you have in mind, there is no symptom too "strange" when making an exclusion diagnosis.
It honestly does sound like depression.
From my experience with my son at the same age what I can say is that unlike adults, its not vaguely a general "malaise". There is usually an identifiable root cause.
From what you say, there clearly is a problem at home, lots of things unsaid and that will involve all of you willing to consider a bout of family therapy or at least a reality check of what you are projecting as a couple, what your DH is expressing (or not) as a father.
I firmly believe no child can be treated without this holistic approach. Don't be put off if you feel under scrutiny because you will be. So better lay it out in the open sooner rather than later.
The good news is that once the root cause starts to unravel and the child feels supported and that there is honesty all round, the anxiety levels VERY quickly go down.
CBT, biofeedback etc work very well and quickly with children. There is no delving into what's fundamentally "wrong" with HIM. Its all about showing the child that there is an objective SITUATION that produces negative emotions and then sort of "re-wire" from there IYKWIM?
Of course there a question of personality type. Mine is an absolute quiet type. Self sufficient, bookish, gets sensory overload.
He is also an absolute star on the field with his team. If you know only side of him you would never guess the other one. The constant in his personnalilty is his resilience and his ability to find the space he needs to regroup.
Its ok to be an introvert. Its the rest of the world that doesn't seem to agree. Sod them!
As an adult, I would feel really awful being pigeonholed as the weirdo as he seems to be. In his own family.Even if its affectionate, its very wrong. How could he possibly feel safe coming home with a grievance if he is de facto shut out?
So sorry to rub it in but something's got to change at home to get through to the other side of this.
Get your husband to read this book.
He will probably learn a thing or two to help boost your son's self esteem and maybe even acknowledge some truths about himself?
Keep talking, I know its a very lonely place to be especially if you get locked into the school refusal spiral.