I actually knew someone like this in high school. His name was Brandon. Brandon was borderline retarded (literally). It wasn't so bad that he had to constantly be in special ed classes, but often enough and he could go there any time he got confused in his remedial classes (frequently) for help. The world around him didn't confuse him. He could drive a car passably, read and write enough to get by (barely) in our world, which is much more literate intensive than most fantasy worlds. He just couldn't grasp mentally challenging concepts like math or physics, not the math induced "you push a frictionless box over a frictionless table at 1.3N physics but the basic physics we all take for granted as children. "Better not jump off the roof because things fall and it'll hurt." That was his 6 Int
His 8 Wis was as many described. He had next to no attention span. Any time you tried to talk to him, especially if you moved just too far to the right on that intelligence border, he'd start grinning and tell lame "You might be a redneck jokes..." and laugh his ass off. He was also very self-unaware. He didn't understand his emotions or how to properly act on them. He was usually really nice, but an angry Brandon was a frightening thing to behold since he weighed in at nearly 300 lbs and played on the Varsity football line. He missed simple details about life and walked about in a daze.
Finally, his charisma was low. Strangely, I've always thought of Brandon as being the epitome of gregariousness. He was outgoing, usually very happy, treated everyone the same. However, he never bathed, ever. He smelled horribly. His arms, each the size of a side of beef, would be used to slap you across the back everytime you encountered him which hurt and put you perilously close to his oderiferous armpit (and pit is an apt term). He interrupted people all the time, made crude jokes at innapproprate times, et cetera.
I agree that he wouldn't be much fun to play, but if your player is still unconvinced, have him read this about Brandon and see what he thinks.
Z