He's at -5. He also falls unconscious and assumes the condition "dying". You can go under 0 hp, when under 0 hp you are unconscious. You die when you reach your negativ CON (aka, if his CON is 10, he dies at -10).
The condition dying:
Dying wrote:
A dying creature is unconscious and near death. Creatures that have negative hit points and have not stabilized are dying. A dying creature can take no actions. On the character's next turn, after being reduced to negative hit points (but not dead), and on all subsequent turns, the character must make a DC 10 Constitution check to become stable. The character takes a penalty on this roll equal to his negative hit point total. A character that is stable does not need to make this check. A natural 20 on this check is an automatic success. If the character fails this check, he loses 1 hit point. If a dying creature has an amount of negative hit points equal to its Constitution score, it dies.
