Atticus's Momma --
As awful as everything feels right now, you can be assured that the REALLY important things are going well -- because your baby's diaper output is fantastic and he is gaining well. I should have also asked you what his lowest weight after birth was (babies normally lose a few ounces in the first few days), but without even knowing that lowest weight, I can see that he's gaining almost an ounce a day. (25 ounces above birth weight in 4 weeks.)That is a WONDERFUL weight gain. It's when newborns are gaining less than half an ounce a day that we really start to worry about them.
The skin rash, frequent fussiness, tummy pains, green stools -- it's starting to sound more and more like some kind of allergy or sensitivity, to either the formula or something in your milk. The most common culprit is dairy, so I think you got sound advice from the LC about eliminating dairy from your diet. If you can also stop giving formula, then that will make the puzzle a little easier to solve by removing one more possible irritant from his diet.
I urge you to keep posting on this forum for emotional support, but also to post in the forum titled "Allergies and the Breastfeeding Family." It is well down the page in the main index. Brittan and many other moms here have dealt with the whole range of allergies and sensitivities, and they'll be able to help you figure out what is going on as well as give a lot of good tips and advice.
If this does prove to be a dairy sensitivity, then you can be encouraged that the changes to your own diet will probably be temporary. Many babies who start out with these problems outgrow them, as their gut matures.
Hang in there! Let us know how you're doing.