I don't know if this is the exact stuff he drinks, but Amazon gave me this:

https://www.amazon.com/Practical-Belch-Powder-Bubbles-Drinks/dp/B004CSZD3U/ref=sr_1_1?crid=3H7DUQ2WGKQUJ&dchild=1&keywords=burp+powder+prank&qid=1625445126&sprefix=burp+po%2Caps%2C171&sr=8-1

The ingredients are on the second image, and half are saying it works, the other say it doesn't.
Good luck.