2001 Jeep Cherokee cold air doesn't blow out

Hi! My son's Grand Cherokee air conditioning has had issues since we bought it. We took it to our mechanic because it didn't blow cold air from the A/C and he said it needed a freon charge (which he did). A month later, same issue, he checked for leaks and could find none, but he said it was a little low on freon so he re-charged it and it seemed to run fine. NOW a month later, A/C not blowing cold again - what could be the problem???

Thanks!!!