Long story short: I thought a female rabbit was a male and (stupidly) let my buns in together for some play time. (don’t worry, they’ve been separated now!) She gave birth to five kits 2 days ago. That’s quite a large litter, considering she and the father are Jersey Woolies.
All was well until this morning, when I found babies strewn about the cage. All were dead except one. I scooped him up straight away and I could feel he was extremely cold and he was skinny and obviously not fed for some time, so I proceded to separate him into a basket filled with carefresh bedding and lined with a freshly warmed towel. My dad and I ran out to a farm store and got some KMR (kitten milk replacer) and I’ve been feeding him this via syringe. I’ve tried three times, and each time he’s managed to swallow a few mil. each time only. He’s had a really big pee after I stimulated him to do so, but he hasn’t defecated at all.
My main questions are: is it okay he hasn’t defecated? And will he be warm enough tonight in a towel lined small basket with bedding in it say atop a heating pad on the low setting? He obviously has no littermates to help warm him. 
I’m putting so much effort into this little guy’s well-being and I really hope he’ll be okay. Any help would be GREATLY appreciated. Thanks for reading <3