I.T.
Mint tea. "Hierba Buena" or spearmint leaves can be taken fresh and boiled- add a bit of sugar. It soothes the tummy. However if it's been going on for a while, you should contact his pedi.
P.S.
He could also chew and eat the spearmint leaves if he's anything like my 16 month-old who will eat anything I tell her not to- including grass.