9 week old kitten with blood in poo (which is greenish and runny) What could it be?

I got a 9 week old kitten last wednesday, When we got him we were told that he had lived outside since he was born because he was so timid. He is now better with us, But im worried as i have just found some poo (on the bottom of my curtains!!) that had bits of blood in it and it was greenish and runny. (I think hes the 'runt' of the litter.) What could it be? And how much is it going to cost to get him checked out at the vet? And how much are the vaccines?