Need help? Possible 2 wk old kitten problem??

Recently a stray kitty had a litter of kittens on my back deck roughly 2 weeks ago. She has since abandoned the kittens and my husband and I have been bottlefeeding them for about a week now. We have been weighing them every day to make sure that they are eating enough. We always let them have as much formula as they want. They currently reside in our bathtub on a fleece blanket with a heating pad under it to keep them warm. This afternoon I went upstairs to feed them and one of them is just like a tiny ragdoll. He ate a little bit but refused the bottle after a few seconds, which is unusual... they tend to latch on and not let go until they are done. He did manage to poop all over hi lower half & I got him all cleaned up but he's just limp. The other kittens are full of energy & climbing all over each other and he just lays there. I called my vet and they seem to think he just hasn't had enough to eat today. Has anyone had a similar experience? Any advice is appreciated!