Patrick Sanders, 20, was shot in the face as he tried to retrieve the firearm from the three-year-old boy.
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The gun had fallen from his pocket after he fell asleep on the sofa, leaving the child to pick it up. Patrick is said to have awoken just he went to grab the gun.
It is unclear whether the shot was caused by the toddler or Patrick. The incident took place in Houston, Texas and police there said: "At that time, the gun went off and Sanders was shot in the face.
"The child was not injured."
Patrick remains in a critical condition after the incident, which happened on Saturday morning just after 8.30am.