Baby poop help - possible milk protein allergy?

Hi everyone,

So up until this week my 7 week old baby had totally normal, yellow soft poops about 3 to 4 times a day. We were combination feeding with approximately 40% pumped breast milk and 60% formula (Lebenswert - for those unfamiliar it's a European formula). My baby was seeming a little gassy and fussy, and spitting up quite a lot, and a mild red rash on his neck which may have been eczema but didn't quite look like eczema. So my pediatrician recommend switching to a hypoallergenic formula and eliminating dairy from my diet, both of which I did about 2 weeks ago. We started him on HiPP Comfort which has extensively hydrolyzed milk and decreased lactose content. Suddenly at the end of last week my baby stopped pooping as much, going 3 days without a poop, and on the fourth day (Monday) I called the pediatrician, who recommended a small amount of apple juice to get things moving. However about ten minutes later before I even had a chance to give the juice, he had a very large but runny greenish-brown poop. It wasn't totally watery, more of the consistency of a watered down, thin BBQ sauce. I didn't think much of it, figuring it was a weird color and consistency because he went so long without pooping. Tuesday he didn't poop again. Wednesday he had another of the same greenish runny poops. Thursday nothing, and now a third runny greenish poop today. Now that we are on our third runny poop I'm getting concerned. Of course my pediatrician isn't in on Fridays or the weekend so I can't call until Monday.

As of this past week or so I've gotten up to 50-50 breast and formula. I'm having serious supply issues. 

Otherwise he seems very happy, rarely fussy. He's sleeping well, gaining plenty of weight. He eats about 4 to 5oz every 3 to 4 hours, and goes about a six to seven hour stretch overnight. He does still have that weird rash on his neck, it hasn't gotten better or worse. I've attached a picture. I don't know if it's eczema? His spitting up has greatly decreased for sure.

I'm pretty torn because it seems like he's happier and more comfortable on this hypoallergenic formula, and definitely spitting up less, but now he's having digestive issues. But on the regular formula he had great poops but lots of spit up, gas, and discomfort.

What could be causing the greenish runny poops? Does it sound like a cow's milk protein issue? Or something else? Any thoughts or ideas at all would be appreciated!!