Food pipe shrinkage after acid ingestion, is usually seen after about 3 to 4 weeks of the ingestion and in about 60 to 70% of kids.
Give him bland food as advised.
He would have been given medicines like antacid
gels. Give those medicines.
If he has repeated vomiting
, its better to see your doctor again.
Hope my answer helps you.
Wish a speedy recovery for your cousin without any complications.