I seem to remember that he made it out of the blanket he was wrapped up in as a baby when he was launched from Krypton?
The jeans would burn off the first time he flew really fast! Lex Luther would only laugh when he showed up.
Freegards
I remember something about the baby blanket too, but it doesn’t jibe. When have you ever seen a baby blanket in three colors large enough to make a costume out of and that had big rubber patches big enough to make boots from ? For a while there was an alternate explanation — that his body produces a force field near his skin, extending his invulnerability to his clothing. But they did go back to the baby blanket story eventually.