is it possible for my son to be allergic to whole milk when he never had a problem with formula? Isn't formula cow's milk based too?

We've been doing half and half for a few days now and his arms have rashes and his face get blotchy and then goes away. The thing is that it is normal for him to get heat rashes on his arms so I can't tell if this is from the milk or the heat. i'm stopping the milk until we talk to the ped. tomorrow.