Interestingly, my kid who was what my father called a “money player”, meaning that when he was performing, or auditioning, he always came through, said he’s getting more nervous as he gets older (and now has no audition stress). If it’s really bad, he could try propranolol, which blocks performance anxiety. He’d have to of course try it out a number of times first. Anxiety Medications – Anxieties.com What To Know About Using Propranolol for Performance Anxiety - GoodRx
Read the above, then talk with his pediatrician right away to see if he/she agrees to your son trying it.