Kitten is not pooping please help =(?

I have a little kitten, about 6 weeks old. eating kitten food just little amounts, and drinkiing formula once a day (on its own). He is peeing, but not taking a poop whatsoever. His mom died, so that family had hiim gave him away. so how can I help it poop?
His tummy looks puffed and feels kind of hard..What do i do?
He hasn't pooped in two days...only urinated twice
also, he's playing and jumping normally...sleeping normally, eating and drinking normally. Nothing seems to be wrong except for his tummy...