Look through his cage carefully. Last week, I was very worried that Kismet hadn't pooped in her litterbox. There was pee, but no poop. There was none in her bag, or in her tube, so i figured she just had an off night and let it go. The next night, there was still no poop. I took her out and stuck her in a warm bath to get things moving (because I was sure she was constipated) and sure enough, she pooped. I figured all was well and put her back in her cage when she was dry. She didn't poop overnight, which was understandable... and then didn't the next night either. The following day was cage cleaning day (I clean the cage completely every week because they're all litter trained and never mess the liners) and wouldn't you know it - she'd decided she liked to go in the corner of her cage beneath her liner. There was no smell, so I didn't notice. I check every day for "gifts" now and there's a fresh one every morning.
So look around. He might be hiding his little presents if you never see any on his wheel. He might be one of the rare ones that doesn't poop on his wheel and opts for a different spot.