"Kindness" by Lester del Rey, first published in Astounding Science Fiction, October 1944, available at the Internet Archive. Does any of these covers look familiar?
In del Rey's story, Danny Black is the last surviving Homo sapiens in a world of supermen:
Homo sapiens! The type of man who had come out of the caves and built a world of atomic power, electronics and other old-time wonders—thinking man, as it translated from the Latin. In the dim past, when his ancestors had owned the world, they had made a joke of it, shortening it to homo sap, and laughing, because there had been no other species to rival them. Now it was no longer a joke.
Normal man had been only a "sap" to homo intelligens—intelligent man—who was now the master of the world. Danny was only a left-over, the last normal man in a world of supermen, hating the fact that he had been born, and that his mother had died at his birth to leave him only loneliness as his heritage.
Danny does not think he is a genius; he is painfully aware of his inferiority, although the supermen go to great lengths to treat him kindly and give him a make-work job. In the end, he thinks he has outwitted them by stealing an old rocket ship and escaping into space:
The note might be ten years or half a dozen centuries old—but his people had been here, fighting on and managing to live, after Earth had been lost to them. If they could, so could he!
And unlikely though it seemed, there might possibly be more of them out there somewhere. Perhaps the race was still surviving in spite of time and trouble and even homo intelligens.
Danny's eyes were moist as he stepped back from the door and the darkness outside to begin cleaning his new home. If any were there, he'd find them. And if not—
Well, he was still a member of a great and daring race that could never know defeat so long as a single man might live. He would never forget that.
Meanwhile, back on Earth:
"Well, he swallowed our lies and ran off with the ship we built him. I hope he's happy, for a while at least."