I'm a vet nurse, I work in a specialist centre now in Sydney and an Emergency Critical Care centre...

I would take him in. There are a few reasons a dog won't use the back legs, some are internal reasons, neurological, toxins and spinal injuries. It could be arthritis, but in a 10 month old, that would be a far fetched idea.

Check him for ticks, I know its not tick season, but we have seen a few nymphs around that have been able to infect animals with toxins... make sure thats not it.

Staffy's are pretty robust and often will suffer in silence, so please take him in.