I think there are alot of misconceptions when it comes to babies and allergies. Lots of parents wind up stopping nursing and putting htier babies on expensive formula when they didn't need to becaue thier doctor told them it was a dairy allergy. Not saying its not uncommon, it for sure is, but alot of times the expensive formula isn't necessary.
My oldest has a dairy protein allergy that I believe was triggered by formula the nurse gave him in teh hospital. Obviously I was angry about that to begin with, he was to be exclusively breastfed, but that one intro triggered his allergy, and it came out in raging eczema that has taken us 6yrs to get under control, find the problem, and alternate ways of dealing with it.
I breastfed him for a long time, and he can tolerate some milk, but ice cream does him in. He can have cheese and yogurt because the proteins are so broken down that it doesn't affect him. We use alot of almond milk, and we all get our calcium the way the cows do... from greens. Milk is not necessary, its not a staple food especially now that its so overprocessed, adn children are being given 'inferior' food (formula) from birth, thier bodies just can't handle it.
So to answer your question, you can continue spending money on the expensive formula, or you can move to goats milk (which is the closest to human milk), almond milk, or skipm ilk all together and keep your son on a colorful, well balanced diet to meet his needs.