Completely agree with @bunfighters - once I”d read your first post, OP, I thought that DS14 probably has similar challenges.
My autistic DS took years to diagnose because he’s so good at articulating everything, and has extreme anxiety, but he also shuts down because of ADHD behaviour. I’m NOT saying, my child, therefore all children, but I really think that, given you have already said you have a diagnosis, and the consultant actually gave you an opportunity to explore it, that it needs following up again. I’m sorry.
I would also suggest that, given your DS14 having such strong reactions to the idea, his entire anxiety/issues with eating etc maybe around the fact that he sees his brother, sees your struggles (assuming it makes life interesting, as it does for me?!), and is desperately worried that he has similar.
In fact, the more I think about htis, the more I see similarities. My allegedly neurotypical child is also a teenager, and finds anything that may suggest he is like us very difficult to handle. Not because he’s awful, but because he’s scared.
I’d put money on it, your son has held it together for years, and it’s falling apart now (very common with kids who have got by on masking and compliance, for it to become too hard as a teenager), and now is having a crisis of confidence/identity. (And by identity, i mean is he neurodiverse, what does that mean, can he “keep being normal”).
Even if he won’t talk to you about it, I’d definitely flag it to whoever is responsible for the enhanced medical and social support.
Good luck, horrendous situation for you, but I hope this is the beginning of solving the most difficult issues.