I can't give a clear answer, but I think there is a degree of difference between Common Knowledge and Intelligence. I believe that being stupid would qualify more under "Thinking Ahead" which comes under Intelligence and Wisdom. Therefore, I'd reduce the Intelligence and Wisdom scores, and deny him the Diplomacy, Insight and Bluff skills, and say that the other skills under Int and Wis needs to have some form of in-character experience as a reason for the knowledge. That is to say; if you perform an History/Arcana check it must be something the character has experienced or heard directly as he would never have read a book.
Religon is a pretty wide subject and I'd say that the character can have general knowledge but no depth to that knowledge (since its such a huge part of the D&D world).
Heal, it doesn't really require knowledge of the body's structure in order to understand an injury, how to treat simple injuries and what herbs help against what. But I'd say that the character would need to have some form of practical experience with it.
Dungeoneering and Nature, same here- experience, I would have the Player explain how the "stupid" Character has experience in the matter he is checking against. (Simple Example: Nature check on animal; Reason - Have fought such an animal before)
Perception, I'd say that perception outside of combat should either be denied or given a severe penalty, but inside combat he'd have normal perception (perhaps even a slight boost since as a fool he'd often have to rely on his 'feelings' for things).
Streetwise is more along Intuition, and fools tend to act on what they Feel rather then on what they Think, so I wouldn't penalize or deny rolls on this skill unless it would determine some kind of deep thinking.
In situations I guess that the 'Stupid' Orc would kick in the door rather then making a perception check or does that fall under 'Air Head'?
