Constipated 1 week old kitten

Hello. We had a cat birth a litter of kittens 1 week ago today. She rejected one of the kittens and would not nurse it or care for it. We had to remove the kitten and have been taking care of it. We started the kitten on formula and have been mixing it according to the manufacturer. The kitten has double its weight in 1 week. It has only pooped once though in this whole week(the poop itself was a massive one. About the length of a q tip). It’s been about 5 days since the last poop. I took it to the vet and they said keep stimulating and it will poop. I’ve tried every which way and have seen a ton of videos and still nothing. Any word of Advice?

5 days without faeces isn't normal; while there are causes (poor or absent stimulation, or low residue diets for example), it doesn't sound like that's what you're dealing with. If he's doubled his weight in a week, that's great; however, it does suggest there may be some underlying medical problem. I'd suggest another trip to the vets - he's big enough now the vet will be able to have a good listen and feel of his tummy, which may shed some light on the situation. Good luck!