Hi I can fully understand your worry. It would be better if you had attached a photo of same.
Keep a watch on the spot... if it increases. Or changes its texture..
Vomit is quite normal in kids.
If his weight and growth is normal.. don't worry.
Next Steps
I have made a dedicated video on topic.. colic in babies.. which includes how to burp baby which prevents vomiting. please watch it on my vlog.... for full answer to your query.. I am sure you will be satisfied.
https://youtu.be/dSkiu0MvMJo
MY BABY CRY'S IN EVENING... EVENING COLIC! HOW TO DEAL WITH IT. Full guide for new moms and dad.
If this link does not open search my channel in youtube..DrPoonamsvlog Also stay connected and Subscribe to me on DrPoonamsvlog on YouTube for more baby and child health information. Hope to see you as my extended family