One of my feral kittens from last year is exactly like that. He was rejected at birth and left to die by mum. We scooped him up and hand reared him.
Eventually he was adopted by the daughter of our welfare officer. He was nearly 8 months by the time his bits were big enough to come off, bless him.
We realised, in our puds case, that mum must have known there was something wrong with him so that's why she left him. The vet thinks he may have pituitary gland problems. Whatever is the matter with him he's a happy little bunny, but very, very small and very, very clingy and won't leave his meowmy's side.
He must be nearly a year now and still acts like he's 12 weeks old.
Maybe something like that happened to your pud and that's why his 'breeder' ditched him on a passer by? It's a lot of work feeding a 'special' kitten and giving them extra support.