D, could he have picked something up and eaten it without you seeing it happen?
I think poo can be so many surprising colours. Today I cleaned up a (sick baby) poo that was chockers full of seeds, even though she'd only eaten 1/6 of a piece of grain toast - there is NO WAY she could've ingested that many seeds... But there they were.
If he seems happy and not bothered by it, I wouldn't worry...