I have a sore bruised spot on the mid sole section of my foot. what is the possible cause.

Many possibilities: Your pain could be as a result of many different things. One would need to do a thorough history and physical exam and possibly some laboratory and radiology exams to determine the exact cause. Have it evaluated by a professional to see if you have fractured it of if it is just a soft tissue injury such as a sprain. You may have a ligamentus or a tendon injury or even a fracure.

Plantar fascia: May have a tear in the plantar fascia or plantar fascitis.

Bruise to foot: The most likely cause would be from trauma or stepping on something. If he does not remember any type of injury to the foot, it ma may have been from a broken capillary on the bottom of the foot leaking into the skin. If it does not improve, he should see a specialist to have this examined.