Lennie, is he mentally disabled?

Why is Lennie so forgetful?

     I realized that throughout part two of this charismatic book, that Lennie seemed very forgetful.  Lennie has to repeat things multiple times, just to remember.  Like when George tells him what to do if he gets into trouble.  George affirms, "'Hide in the brush by the river.  Say that over.'"  I beleive that Lennie is mentally disabled, because he always forgets things.  Also, he, not meaning to, gets into trouble.  He also has quite a mind - boggling amount of childish behavior for his age.  That corroborates (ding) my theory of his disability.  But, just because of his disability, or so I think, he is still very sweet, and loves soft things.