Hi, Welcome to Cat Forum and special hugs to you for rescuing this little baby. You've only had him a few days, and it is possible that he'd had no opportunity to nurse and his current nourishment is being utilized for his body, and is intestines are just slow to move things out. Keep stimulating him, eventually he will have to go. Food in = stuff out.
It has to come out sometime.
When I got an orphaned litter to care for, I was having difficulty getting them to poo and one of my male cats actually helped me. LuckyDuck(Louie) *loves* all kittens/cats and he was always very interested when I cared for the kittens. I couldn't get them to poo, though pee was definitely not a problem. Louie was in the room with me, and he began poking around the kittens, he rolled one onto its' back and began licking its' belly and the kitten poo'd! He started on another one and I grabbed the 3rd one, gently rolled him over to his back and rubbed with the cloth and he poo'd, too! Maybe give that a try?
If the kitten's belly does not get 'hard' distended and it shows no signs of distress, just keep trying. If you get concerned about the baby, I'd take it to the vet for an exam and advice. Little kittens are so small that if/when things go wrong, it goes wrong very badly and is extremely difficult for the little body to recover on its' own.
My favorite kitten milk replacing formula is called "Just Born with Colostrum" and is made by Farnam. It is available in small containers already mixed up or in a canister of powder that you mix yourself. I used the canister for litters or single orphans I got at a young age and would require weeks of bottle feeding. Your kitten could be around 8-14dys old, due to the half opened eyes and will require bottle feeding for at least 7 weeks. Kittens can be introduced to canned food around 4wks, but some don't show interest until about 5wks of age and even 6wks, for late-developers.
Best of luck with the little one!