Hello, thanks for using HCM.
There are two main possibilities for his condition; it's either he has a food poisoning
or he is allergic to some stuff in the food he ate.
Food poisoning is a common following dishes including fish, milk and usually affects a handful of individuals who ate the same food. However, though it could present with severe abdominal pains and bowel motion, it usually clears off within a few days on conservative management such as fluid replacement and pain killers
On the other hand, food allergy
occurs mostly in people who are predispose or with known history of allergy and usually is due to particular food stuffs.
For now, you should increase his food and fluid in take, give him banana and carrots to harden his stool, and if he is not better within a few days, or if his bowel motions become so frequently or if he runs a temperature, you should consider taken him to see a doctor
Best regards and hope this helps.