My 8 week old kitten hasnt pooped on 2 days. Is there any advise

I would get him some wet food, and put a few drops of olive oil in it each time you feed him. The oil won’t hurt the other kittens either. At 8 weeks, he should be using the litter box. Another thing you may want to do for a little while, is switch over to plain, clay, unscented non clumping litter as sometimes kittens eat it and there is the danger of an intestinal blockage. I would limit the dry food until he poops, as it sucks the water from their bodies even though it’s convenient to feed. You could also get some goat’s milk and mix into his food also, it’s full of probiotics and good for their gut.