I am sorry about the issue you are dealing with in your young child. We do know what lactose intolerance results from - a lack of enzyme that is needed to break down lactose that is a sugar found in milk. Unfortunately, we do not know why some people are lacking that enzyme. There are some hereditary patterns - some parts of the world for example have a higher prevalence of lactose intolerance. Sometimes people are born with the deficiency and their issues with dairy are noted early in life like in your child and other times people develop the deficiency over time - as they get older. At times the progression is sudden (people develop issues with dairy quickly) and other times gradual and slow - people notice the GI symptoms slowly creeping up on them. It is not clear why the enzyme's production diminishes or is completely lost in those people - at times that is a result of a GI virus, other times prolonged illness and use of antibiotics and other medications for long periods of time but the reality is each case is different. Lactaid products are rarely helpful and I generally do not recommend their use in my patients.
I would like to reassure you though that your child will not be lacking nutritionally because of his lack fo dairy consumption and will grow and develop without any issues. Dairy by itself is not a requirement for human development. Calcium, fat, and protein, which are found in dairy products is what humans need, but the reality is they can find those nutrients in many other food groups. Fresh fruits and veggies, eggs, legumes, meats, and seafood are all good sources. There are a variety of non-dairy milks currently, which can provide your son with what he needs - combination of tree nut milks (almond, cashew, hazelnut, etc.), coconut milk as well hemp and pea protein milk are all excellent choices.
I hope that helps and relives some of your angst.
Monika Korff, MD