Found an orphaned kitten please help!!!??

I heard a meow around my back porch for about 2 days but couldn't find it.. I went out the door on day 2 and happened to finally see it on my way to the car.. I left it there thinking momma would come get it but when I came back it was still there and barely making noise.. I picked it up and it was so skinny I'm surprised it hadn't died. It wasn't moving much either and its eyes were still closed with gooey green stuff on it..

Anyway, I've had it for a couple days now and bought milk replacer. I've also been stimulating it to potty. I'm having a hard time getting it to stay still when I feed it so the milk goes down its neck instead of in its mouth mostly.. it also pees every time but I havent gotten it to poop at all..

My main questions are how much should I be feeding it at once and any tips to make it poop? I'm getting worried about the fact that it hasn't at all.. I really can't tell how old it is but it can't be very old at all bc it's eyes mostly stay closed.