A boy is born [possibly in the American mid-west], and before long his parents realize that he is mentally retarded. He makes no attempt at speech. At about the age of 5, his father takes him to a pet shop, hoping that a pet will 'engage' the child. While at the shop, the child suddenly announces 'that one', and points to a particular pup. From then on, the child and the dog are inseparable, and the child rapidly develops mentally, possibly even exceeding 'normal' expectations. Years later, when the boy is perhaps in his late teens and the dog is very old, a situation develops in their small town, wherein a swamp close by is to be drained and back-filled for development. It then transpires that the swamp is in fact the location of a crashed space-ship which is lying buried in the mud, and which is the location of a very long-lived alien who, unable to escape from his stricken craft, has projected his mind out of the ship into the mind of the nearest entity which, it turns out, is not the boy, but the dog! Apparently the alien had hoped to find an intelligent mind, so that the owner of that mind could be directed to take steps to extricate the stricken spacecraft, but had either been repelled by the retardation/delay of the boy's mind, or had 'missed' and entered the dog's mind instead, and had been controlling the boy from within the dog's mind, hence the abrupt changes in the boy's apparent intelligence. The boy, now a young man, must take measures to release the 'craft before it is buried and the trapped alien dies within, while simultaneously coping with the possible reactions of the townsfolk and the powers-that-be in the town, especially the paranoia. The young man does have success in freeing the trapped alien and, before leaving Earth, is able to correct the fault]s] in the young man's brain which were the cause of the retardation/autism/delay and which the alien over-rode through the agency of the dog.
Pretty bizarre? I think I read this sometime in the period 1960 to 1964. It has been suggested to me that the story may have been Theodore Sturgeon's 'Tiny and the Monster', but I am a fan of Sturgeon [as well as Clifford Simak] and know that his short story is not the one I'm looking for. Also, I entertained the idea that it may have been 'A Boy and his Dog' [Harlan Ellison, I think], but I think I was clutching at the straw offered by the title. The theme of paranoia seems to have remained strong in my memory of this story - that seems to suggest the story may have been written in the [fairly paranoid!] 1950's. I don't know if the piece was a full-length book, or a short story within a collection of works by the same author, or a short story within a collection such as 'Astounding Science Fiction'. My father read a lot of sci-fi borrowed from the town library in the small town where we lived - it is perhaps possible that the story may have been contained in bound collections of popular sci-fi publications.