Even though he's weaned in that he can eat solids, if he was still with his Mum he'd still be having milk feeds too, he's probably constipated poor little boy. Some oily fish as S suggested, also add some extra water to his wet food to increase his fluids. A wet diet will keep things moving better than dry, he is very young and his system is stll very immature. He will only have been eating solids in any real quantity for 2 - 3 weeks so not surprising his tum is struggling without help from Mum .
A vet check is a really good idea, vet will advise on fleaing and worming such a little kitten.

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