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

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

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