My boyfriend told me that every time after he gives me oral sex he gets a canker sore, we have both been tested and are negative, what can be the cause?

Stress etc.: Cold sores are a result of a virus that is already in his "system" and comes out as a result of stress. Canker sores may result from localized trauma/injury, lowered resistance, lack of sleep, etc.

Local trama: The activity of oral sex may provide enough trama to inner lip or cheeks that may trigger an outbreak of a canker sore or apthous ulcer. These are not infectious or caused by bacteria or virus, just body reaction to some trigger, like stress, trama, certain foods or food coloring or flavorings, etc.