You do not have to use whole milk. I always recommend it because it adds to the creaminess. You can use other milk, like 2% or fat-free. If needed, you can even use almond milk, but you want to be careful that it isn't too watery when you add it. It doesn't get as creamy as regular milk does. In my opinion, while you can use other milk products, this is one of those dishes that you want to splurge on whole milk if you can.