Ball python suddenly not eating, losing weight. Worried!

[Mayhaps]

So, I have had my regular male ball python for about 6 months, since June. I don't know his exact age, but I'm guessing he's around a year old. He has always been a healthy eater. I have always fed live. I always make sure he has caught and killed his food before I leave him alone, to make sure he does not sustain an injury. He always eats immediately after noticing the food item. I know it's normal for ball pythons to fast during the winter, but I am concerned about his drastic weight loss over the past 10 days. It seems like his skin is kind of loose around his body. I can pinch his sides a bit. He's not lethargic. I'm only worried because I can tell he has lost weight. I'll give some background so you guys have a better idea of what may or may not be wrong.

For the first 2 months I had him, he lived alone. He had the exact same setup before, except he was in a 20 gallon and now he is in a 40 gallon. Then, a couple of months later, about mid-August, I got a male pastel ball python that was around the same age. He is 2 or 3 months younger but the two are almost the same size. At the time, I didn't have another tank to house the new addition in, so I put them together. I was going to separate them once I got a bigger tank, but they seem to enjoy living together and seem super stressed when they are apart, roaming until put back together. After I got the new snake, I continued feeding my first snake on Fridays, and decided to feed the newest on Tuesdays. I always take the one that is not eating out of the tank and just drop in the food item so that the snake doesn't see me drop it in and therefore gets to hunt a bit. After they are done eating, I keep the non-eating snake out about 30-45 minutes, until the freshly fed boy has settled down and snuggled into his hot spot. I have never had a problem with either of them eating. Both of them have always eaten right away and they have never acted aggressive towards one another afterwards or between feeds.

I have them both in a 40 gallon terrarium with a screen top. The humidity stays anywhere between 45%-70%. It's difficult sometimes to keep the humidity above 50% because of the crazy weather in Louisiana during the winter. I have almost alleviated this by covering most of the screen top with plastic wrap, but sometimes I have to mist the tank a bit. Ambient temperature is between 71-80 degrees. I have a UTH under the warm side that stays on a thermostat at 91 degrees. During the day I turn on a basking light so that they have the choice to come out if they want, but I know they are nocturnal so this doesn't happen much during the day. It's mostly for light. I have identical large hides on both sides of the tank. I recently switched from Repti-bark to Jungle Mix substrate which helps me with retaining humidity but I did this after his first week of not eating.

I have tried both rats and mice. He was eating 2-3 mice per feed, though the last time he ate was actually his first rat. He ate it with no issues and had a bowel movement about a week later which is normal . I know he's still urinating because I separated the two snakes to see if it makes a difference in my problem child's temperament, and to find out if he is still drinking water. Luckily, he is. It has only been about 6 hours since they have been separated. So far, they are both incessantly exploring their enclosures.

I was thinking about getting a rat pup today and leaving it in there overnight to see if he will eat it. I don't want to leave him alone with an adult mouse or rat for fear they will hurt him. I'm mostly worried that he has intestinal parasites, but he hasn't pooped in a couple of weeks so I don't have a sample to bring to a vet. Do you guys think he will be okay if I wait another week to see if he will eat and how he does by himself? Sorry this is so long, just want to cover all my bases. I'm so stressed and worried! I have included some comparison pictures so you can see the weight loss. Thanks for any input.

Narcissus 12/20/2013: http://s367.photobucket.com/user/eso...tml?sort=3&o=1
Narcissus 12/30/2013: http://s367.photobucket.com/user/eso...tml?sort=3&o=0