Try to eliminate any possible causes of the problem; dairy products, beans (that one so famous it was one of the themes of a movie!), broccoli, anything he finds difficult to digest. Sometimes, taking digestive enzyme supplements with large meals (not snacks and not usually breakfast) can help. Among dairy products, milk is a particular problem for some because the lactose in it is difficult for adults to deal with. Finally, a lot of people find probiotic supplements (supplements of "friendly bacteria") helpful - but they need to be taken away from food with a cold drink, not hot.
The herbs already mentioned can help too. That's about it for helping the underlying problem, which is digestive problems. For fast relief from the immediate problem, some people find that charcoal biscuits or tablets help. By the way, one inevitable side effect of charcoal supplements is black stools. No surprise there, perhaps, if you're eating charcoal!