When is the best time to introduce cow's milk to my 1, 5 year old child. He still gets a belly ache from milk.?

Try lactose free. At anytime infants can tolerate cow's milk, full strength. If they don't digest it well the infant may be lactose intolerant or he/she may have cow's protein allergy. A lactose free and/or a pre-digested , cows milk protein free formula should be tried.Get the supervision of a pediatrician for these trials.
Alternatives. If your kid gets sick every time they drink cow milk you might consider alternatives. Sometimes its the sugar in the milk (lactose) & this is in much smaller amts in some cheese and yogert.It can be the milk protein, so consider other sources.A simple calcium source are Tums (calcium carbonate) wafers, a couple/day = 4 servings of milk. Water & other fluid/protein sources from the rest of the diet can substitute.