My six month old puppy has developed what initially looked like spots on his chin, now they look more like sores as I think he has knocked the tops off them.  they don't seem to be bothering him, but what are they? and where has he got them from?

Its sounds like acne, mine usually had them during a hormonal spurt. (Similar to teenage acne)
I washed their chins after each meal (and washed the bowls) and left them alone as if mauled with (like us) they can become infected.
Some vets prescribe anti biotics but in our case they would only return as quick as they went after the course was completed.
They still occasionally have them but not very often as they did when youngsters.

Yep mygirls right they are like a form of ache they come to a head and then burst they look much worse then they are, just keep them clean and they'll clean up in there own time.  Good luck warm regards Susan