But he did change the tube ... and the second one went flat too. He needs to have a really good close look at his tire before he puts another tube in there. It could be a tiny piece of glass, or a miniscule wire, or something like that.
To make finding whatever it is easier, the OP should have marked the location of the tube valve on the tire before removing the tube ... then located the hole in the tube ... then matched the tube to the tire ... then looked for the flat cause in the location of the hole. But it is too late for that now, and he's going to have to check the whole tire.