Could a man be proved positive for a pregnancy test?
Pregnancy test, just like any test, has some non-zero false positive rate. So yes, of course a test could be positive when a man pees on it.

Pregnancy tests work by detecting the presence of a hormone called Beta-HCG. This hormone is produced by both genders but is produced in much higher dose during early pregnancy. Teenagers also produce a relatively high dose of Beta-HCG which can eventually inflate the false positive rate.

It just so happen that tumours from a particular type of testicular cancer also excrete Beta-HCG (see this blog post from the American Cancer Society). If your question arises from a personal experience, then I would strongly recommend that you talk about it with your doctor.