M.P.
I recommend going to a pediatric allergist. My granddaughter tested positive for several thinks with thec blood test. She had eczema and asthma as a baby which prompted the blood test. Sometime between 1 and 2 years my daughter took her to the aallergist. My granddaughter dealt well with the prick test. Everyone in the pediatric allergist's office knew how to help a toddler with the testing.
A pediatric allergist has a much broader scope of knowledge to allergies
than a pediatrician. By getting the testing done and help with it's management you will have this under control much faster. Also consider that he may have environmental, not food allergies that can also have a negative effect on his health.
I suggest trying enriched organic rice milk.I'm lactose intolerant and have used it for years. The taste to me is closest to cows milk of all the substitutes. My granddaughter drank rice milk at that age. BTW she outgrew the milk protein allergy by age 6. I buy the rice milk in the steri-cartons on the shelf. I've not tasted the refrigerated kind.
Her little sisters, ages 1 and 3 drink almond milk. I think they buy the vanilla flavored kind. I also like the vanilla coconut milk found in the refrigerated dairy case.
You could try mixing cow milk which he's used to with a nut or rice milk gradually decreasing the amount of cows milk. This works when transitioning to milk from formula.
Anything you make with milk you can make with any of the substitutes. I use rice milk in soups as well as baked goods. You could add calories with cream sauces. You could make smoothies. By blending a fruit with the milk you increase the nutrition and change the flavor.
About dairy products. My granddaughter could eat hard cheeses such as cheddar even tho she couldn't tolerate milk. And there are ice creams made with rice and coconut milk. The coconut ones are best.
I have several food allergies and tried isolating them with an elimination diet. I found doing this difficult and stressful as well as not very effective. To do a true elimination diet one starts with eating just a basic 2-4 foods snd add a new food every 7-10 days all the while keeping a food diary. I couldn't stick with it besides it would take forever to test all foods. After testing with pin pricks I had less foods to test. I initially avoided all the foods that tested positive and soon felt better. Over the years I've found that the initial testing was aaccurate.