contaminated milk

My son has been on Nido since he turned a year, I never had any problems because his eating well and growing well. But since I opened his new tin of Nido his consuming it for a week now I. Noticed he had diarrhea. At first I didn't even think it was the milk but now I do. Coz after a week the symtoms is worsening and he only drinks milk. Don't eat any food or yogarts. I opened one of the new tins I bought and they were bought from the same batch with the number underneath and it's doing the same thing.