It's eth, actually, not thorn.
I had thought that eth was used in Old English for the voiced "th" and thorn for the unvoiced "th", but Wikipedia says they were used interchangeably for both sounds.
You're right otherwise. Thorn was not available on printing presses because they were being made in countries that didn't use the letter, which is why the letter Y was used instead until "th" became more common.
He has a pocket in his cape. It's where he keeps his Clark clothes, shoes, and glasses when he's Superman - super-compressed first, of course.