OT: My infant nephew and Lactose formulas

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Hi,My nephew is 5 weeks old now and I've been keeping a close eye on him since he was even in his mother's womb! He was born 3 week early and weighed 2.5 kg (5.5 lbs).He wasn't gaining any weight on the first 10 days but I stressed a lot about not giving him any formula. Finally after he was 2 weeks old his parents started giving him formula along with breastfeeding because they were sure he wasn't getting enough from his mother. They tried 3 different cow milk formula and all of them caused some intestinal problems (bad smell, greenish color, gas..) I thought about parasites but yet I suggested a lactose free formula, when they tried the LF his gut was good and he gained weight more rapidly. He's around 7-8 lbs now. Now my brother is thinking about trying the regular cow milk formula back a few weeks later. I thought it was a very bad idea that would hurt his gut but I'm not sure if I'm right (?). Currently he's breastfeeding but takes this LF formula with every meal ( about two ounces)My questions are: How can we know for sure if my nephew has a lactose sensitivity? When is a good time to introduce regular cow milk or cow formula that contain lactose?Anything we can do in case he came back positive about lactose sensitivity? Thanks,Noura