If the nappies are wet/full as usual, I'd consider a certain food (asparagus has a reputation for this, for example), or maybe teething? Could be the cold, I know swallowing mucous changed my kids' #2’s whenever they were sick. Medicine can change the smell too. I guess I wouldn't worry too much if I was pretty happy with the amount he was drinking. But you know, usual disclaim, I can be too relaxed about these things so if it really seemed unhealthy I guess you could mention it to a GP or MCHN or something. Hope the cold clears up soon, it can really annoy them hey.