Thank you for your query.
It is very unusual to have cow's milk allergy
with only the first intake of cow's milk. The most likely cause can be an intestinal infection. The best course would be to get the stool examined and give your son some probiotics. Ensure that he remains well hydrated.
I hope I have answered your query to your satisfaction.
Wishing your son an early recovery.
Rajiv K Khandelwal