Seems to me a 9wk old should be pooping at least 2-3 times a day, but if you just got him or he was very skinny and he's making up for lost time he might go less, but once in 2 days seems a bit extreme to me.

If he has a good appetite it's a good sign, but I'd get a check up with your vet anyways just to be sure your new kitten doesn't have any other health problems. Bring up your concerns about his pooping with the vet and a poop sample if you can get one.

Prevention is always less expensive and less heartbreaking than the 'just wait and see' approach.

Good luck and congrats on your new kitten, may you two have a long, and purrrfectly happy life together.