This is a short story from an anthology I read c, 1970-85.
Sometime in the future, probably after some kind of nuclear war, many children are born mutated and monsterous. The government or army rounds them up and destroys them without pity.
A man and his mutated son hide out in a ruined building. The boy has to remain hidden while his father fetches them food and supplies. Unfortunately the father has become sick (radiation?) and is rapidly becoming weaker and near death. His son wants to go for help but his father forbids it, knowing his mutation is already too obvious. He has large muscular growths coming fom his spine that are too big to hide.
Eventually the father passes out (or away) and the boy decides he really has to go and look for help or die trying. He climbs to the top of the building, spreads out his mutated growths and...
...flies. He's an angel.