The „pimple“ is a normal part of the bird’s anatomy, it’s the uropygial or oil gland and it produces the oils necessary to waterproof and care for his plumage - think of it as nature’s conditioner.
Usually the gland is barely noticeable, because it’s hidden in the feathers. I am no vet, so I can’t judge whether the appearance is as it should be (it differs a lot between various kinds of birds). It appears a bit “full” (the cushion like bit, the “nozzle” is fine), which can happen at times. Please watch your foster baby for a bit and if he shows any signs of discomfort or the gland grows, contact a vet. As a general guideline, ensure that he gets all his vitamins (especially vitamin A) and a balanced diet.