I suppose the only likely explination is that he's knocked it but i'm not aware of it. I can't recall any incident involving his mouth and lots of tears. I assume it would have been a knock that hurt him

Raven if it can take some time to discolour like you describe this may have happened some time ago also. Is this something they'll just leave until his baby teeth decide to fall out? Is it painful for them when biting?

I just called a dentist and they have kids checks at a special time each week so i'll take him for a check and then get advice from there.

It makes me sad, i know it's just one of those things that can happen with children but still...