Chances of getting an STD from performing fellatio?

The risk is low, though I'd refrain from doing that in the future. STD's are generally carried in bodily fluids, and the lack of exposure to such diminishes the risk considerably. There are, however, STD's that only require contact with the genitalia, even if no fluid is exchanged (HPV, for instance). Genital warts and are also inherently episodic, and lesions can appear both internally and externally and range from pea size to microscopic, meaning just because there's no visible lesions on or around the doesn't necessarily mean he's clean. To be safe I'd get tested.