How do i keep from getting aids from a guy that already has it? is must be in his saliva too. am i doomed?

Safe sex: You don't have to worry about his saliva. Just don't have sex with him without condoms.

Use protection : It is rare for HIV transmission from saliva, however, if the person is known to be infected and has a high viral load in his blood, and he has severe inflammation of the gums due to periodontal disease the risk of transmission goes way up from kissing. If you want to keep seeing this person then you need to have him share with you his most recent blood test results to help you decide if it is ok.