As a new hamster owner myself I can recognize the worry, but I'm sure as others say - he likely just needs time to settle in and get used to everything new to him, including you.
Luckily mine chose to poop in his sandbath which is easy to clean, but I'm sure he has a proper spot in his main house as well, which I haven't touched for two weeks and won't until I've successfully had him out of his enclosure a couple of times.
I wouldn't worry though, time and patience.
