"Master Race", a short story by Richard Ashby, also my (unaccepted) answer to this old question; first published in Imagination, September 1951, available at the Internet Archive. The aliens find the stuff in the boy's tree house, not his bedroom, and the comic books are unidentified sci-fi comics, not necessarily Superman. Otherwise it matches your description.
Summary from the Recursive Science Fiction site:
Earth is about to be invaded and humanity destroyed. A scout from the invasion fleet steals [the] contents of a tree house and returns for evaluation. Some of the items are science fiction comic books. The invaders, having no fiction in their culture, take them to be historical records and flee in terror.
Excerpt from the story; "Spalding" is misspelled:
His first few minutes of exploration in the tree house were disappointing. There was no life, no corpses about for him to dissect and study. But the hunting club puzzled him. Obviously tooled by machinery and scuffed from much killing, it bore what might be a
word burnt into its thickened end: "SPAULDING." He realized he was in an extremely primitive section of the planet, for this weapon was, no doubt, a trade article from some
more advanced portion of the globe Too bad he’d had to land in this region. Dull.
The club he chucked into the bag over his shoulder.
A round object, made of some fairly soft material, with seams twisting over its surface next caught his eye. He took it up, shook it. It, too, bore the symbol "SPAULDING." Probably a totem word. Perhaps the sign of this particular tribe. He put it with the club. It was followed by a small package of soft white cylinders which were stuffed with crumbles of dried weed. Each cylinder bore the sign "CAMEL" as did the container, which
also showed a beast, somewhat like those he had buzzed.
He discovered the Council was waiting for him. Frowning to cover his embarrassment, he took out the last of the Scout’s finds. For a moment all of them were struck by the bright colors on the flat surface. The one old man reached out a trembling hand. “Records,” he murmured incredulously. “Records such as our own race is said to have once made, long long ago before Law.” Reverently, he examined the cover, then with remarkable agility for one so decrepit he jumped to his feet and flung the thing from him. His face twitched with horror.
The others shocked and disbelieving, fell to examining the rest of the new articles. In a moment, cries of alarm filled the council room. Chairs were upset, dignity forgotten. Only the eldest retained his composure, although with difficulty, for he could hardly manage to control the palsied shaking of his
hands. The astonished Commander leaned over his shoulder and watched as the ancient turned the pages.
What he saw made the blood drum in his ears, made his vision swim, and only faintly did he hear the old one’s croaking words. “Praise to Law, whfch we so carelessly accepted, for Law has saved us from the fiendish denizens of this planet. Had we attempted to exterminate them, their space armadas would have taken instant revenge. For they are obviously mightier than we.” He put down the bright record of space craft vaster than the one which they occupied and took up another. On its cover was depicted a world being blasted into flaming wreckage, and within was shown the pictorial history of a space fleet, engaged in repelling an alien invasion, and who followed up their successful repulse by annihilating the entire system of the aliens.
Five more of the record books did they examine before the Commander’s stunned mind at last reeled beneath the hideous concepts and he could look no more. Dumbly, he managed to reach the phones and order the ship thrown into emergency drive to some far and lost point in space and dimension.
And as he waited for the shuddering wrench that signalled interdimensional shift, he tried to forget the horrors they had so narrowly escaped: Creatures who could make themselves invisible, who had mastered space travel, who worked in magic more powerful than that of Law’s, who could whiff out entire solar systems, who could survive incredible mishaps and hardships. Creatures who were no less
Another story about "invading aliens scared off by kid's comic books" is "Reconnaissance" by P. W. Cutler in New Worlds Science Fiction #30, December 1954, also available at the Internet Archive; but it is a more obscure story (apparently never reprinted) and has no mention of Spalding, so I don't think it's the one you remember.