For many years she worked with Earth's Best Organic Baby Food, integrating nutrition and product development. She has written numerous articles on children's health and nutrition for parenting publications. Lactose intolerance does cause loose stools. It can also cause gas, cramps and bloating because the sugar in milk is not digesting. If so, that would be more of an indication that he is lactose intolerant.
People experience different degrees of sensitivity.
Your son may be free of these other problems. Lactose intolerance does not always require the complete elimination of lactose containing foods. The problem is very dose dependent. Often a small glass of milk eaten with a meal will not cause any problem at all, but two or three glasses on an empty stomach will. Try serving your son small amounts of dairy at mealtimes.