I agree with PPs that your best bet for immediate help is self help.
Teach him breathwork. Slow deep breaths in through the nose for count of 4, hold for 3 out through the mouth for count of 4. Do this 3 times. Relax, then repeat, maybe extending the length of in and outbreaths if that feels natural or easy. Then trying it very silently, so he can do it in public without drawing attention to himself.
I used to very gently insist my autistic adhd son did affirmations with me when his anxiety was at its worst. I'd be clear that he didn't have to believe them yet, that they can take weeks to actually establish as first thoughts, but repetition of useful ones, practiced regularly have a similar effect on the brain as working out does on the muscles. He just needs to say them aloud neutrally, without any scorn in his voice.
I got a LOT of resistance for this, but I made some of them funny by phrasing them in language that wasn't too serious.
To make a start with this, so it feels authentic, you can use bridging affirmations e.g. instead of saying 'I feel calm' say 'I accept the possibility that I can feel calm in the future' 'I would like to feel calm and so I am prepared to try to feel calm by saying I feel calm.' 'Even though I don't feel calm right now, saying 'I feel calm' might help so as an experiment, I will say I feel calm, I feel calm, I feel calm.'
This is the last thing teen boys want to be doing but it does have a physiological impact. I asked DS to do it for 5 minutes and then check his physiology. Was his heart racing just as much? Did he feel just as sick? He discovered he didn;t, and that increased his willingness to try
Give him extra vitamins, especially B complex, magnesium, iron and Vit D as all of these can be deficient in adhd brains. Especially if he's finding it hard to eat. Vitamin and mineral deficiencies can really exacerbate anxiety. DS always had the worst panic attacks when he'd not eaten anything all day and his blood sugar plummeted. Try to encourage him to eat lots of protein and slow release complex carbs. If he can't eat much, make small amounts count. I'd do a small milkshake for DS with full cream milk, a scoop of vanilla protein powder, a teaspoon of nut butter and a small banana blended into it, over ice. Even if he only drinks half of it, that's a lot of calories.