My neighbours, when I was a kid, had a cat with a batch of 3 kittens. Their little boy (6 at the time, maybe?) built a "house" for the kittens in the garage out of old shelving units, matresses, ect. He put the kittens in it, and it collapsed. One kitten got hit by a shelf and sustained brain damage. He never grew any bigger, was slow, seized, had to be warmed and cared for seperately. Eventually he seized and was jsut so damaged the family had to have him PTS. He suffered for several months like this, before they made the decision. It was selfish of them to keep him alive all that time because they felt guilty for what happened to him, and wanted to "save him".
From the sounds of the condition, it is likely that the pup should be humanely PTS

Such a sad situation.