I was told that you need anti virus for mac pro

You do not need any, but... are you really running Mac OS X 10.5, which is the forum you posted in? That's quite an old OS, and you may need to take additional precautions (just not anti-virus software).

He told you that on the hope that he sell it to you and get some additional monies from you.

That is all AV software is good for is put gold in the pocket of those who sell it. It certainly does no good for the Mac it is installed on.