Allergic to Whey...how to find about about casein

[J2]

My son has been on a dairy free diet. He accidentally had something that had whey in it (taco seasoning) and reacted (rash). So now I at least know that he is allergic to whey. How do I find out if he is also allergic to casein? So then I might know if there are some milk products he can have (or not).

BTW, he's almost 3. We are hoping he outgrows this so we have been on a dairy free diet.

[lil_miss_understood]

The only way I can think of to test it is to find something with a sodium caseinate in it... but of course then you'd have to be certain he wasn't reacting to something else in it, yk?