Well, I'm no expert. However I've had severe eczema (just like he described) and it started off as mild
. I'd be cautious if I were you, that's all. I don't think it's coincidence that it cleared it up, especially because it came back again with the introduction of cow's milk. To me that's an obvious sign of an irritant.
I'd probably just keep a close eye on him like you're doing, and if the eczema gets any worse or you see any other symptoms, I'd switch back to the Neocate asap. At least you have ~6 months of your prescription left and if it's going to get worse from the cow's milk, it will probably happen in that time.
Eczema is so yucky. It took me years and years and many doses of prednesone before I found out what was causing mine (which happened to be powdered laundry detergent of all things?!). From having eczema, I wouldn't even want a mild case back. It's soo itchy and sometimes sore.
You are his mom, you know what's best. Follow your gut and just keep your eye on him. You will know if it's time to switch back.
And--I don't think giving milk intended for baby cows to humans is any more "natural" than drinking something else. But that's just my opinion!
Anyway, I hope he continues to not get any worse and if anything, better!