Have the rest of the family conduct a double-blind test. In other words, neither you nor the child now which is which.

Have you tried nut or grain milk? I like oat, soy, and rice, but cashew is pretty good, too.

This might be helpful, or it might be unrelated.

Recently, I made mozzarella from scratch. In order to do that, I needed some milk that wasn't homogenised. Homogenisation is the process of breaking up the fat globules within milk into smaller droplets so they're more evenly dispersed throughout the liquid, meaning there won't be a fatty layer that separates out when you leave the milk to stand.

Most milk that you buy at the supermarket would be both homogenised and pasteurised. I learned that pasteurised milk could work for cheese, depending on the specific temperature the milk was heated to during pasteurisation (because the required minimum temperature for pasteurization is below the temperature that causes issues for mozzarella, but some brands pasteurise at a higher temperature. Unfortunately most brands don't say what temperature they pasteurise at, but I got lucky with the first one I tried). That part's not especially relevant to you and is mostly cheese related

The thing I wanted to suggest, out of scientific curiosity more than helpfulness, is that I wonder how your son would do with pasteurised, non-homogenised milk — perhaps it's the homogenisation that's causing the problem, rather than the pasteurisation. If you do try this, I'd be interested to hear back how things go; I haven't heard of anyone having issues like this before

FWIW, there's a lot we don't know - but are learning - about bacteria and the gut. For example, if I'm not mistaken, a baby gets a lot of important gut bacteria from it's mum through breastfeeding.

So when I hear all this argument about raw vs pasteurised milk, I expect there really is something of health benefit to raw milk, just there's a big downside of harmful pathogens that can be cured with pasteurization. That doesn't mean all raw milk is unsafe. Like with raw eggs in the UK, or not iodizing your vegetables, it can be safer with care over production.

Anyway, that is to say, I figure there could be some interaction with the bacteria in the raw milk helping your son to digest it.

But having seen the other comment suggesting homogenisation, that sounds more likely to me. (Just a guess though.)

You should give oat milk a try (or soy/almond, but oat tastes the best imo)

Should totally stop feeding him raw milk, get those lactose-free milk instead.

Maybe don't fucking feed him milk at all? Whoa mind blown...