According to federal government statistics, about 1 out of every 100 people is mentally retarded, although some place this figure as high as 3 percent. Mental retardation is slightly more common in males than in females. It occurs in people of all racial, ethnic, educational, and economic backgrounds. For more information, go to the website of the American Association of Mental Retardation.
In contrast, a learning disability (LD) is regarded as a disorder in one or more of the processes involved in understanding and using spoken or written language. Learning disabilities show up in different ways in different individuals. They can have problems with visual perception (understanding or remembering what they see), which can make activities like reading letters or copying shapes very difficult. Or they can have problems with auditory perception (understanding or remembering what they hear) or using language to tell or write a story. A learning disability can cause difficulties in math, reading, writing, or spelling. Some people with a learning disability have organizational problems that can affect school or work.
People with learning disabilities generally have average or above-average intelligence. Their learning disability, however, creates a gap between ability and performance. They tend not to do well in environments that aren't suited to their learning style, but they can learn very well when taught appropriately. Learning disabilities often run in families. Fifteen percent of the U.S. population, or some 39 million Americans, have some form of Iearning disability. About 50 percent of all public-school students in special education have learning disabilities.
LD and MR are two distinctly separate conditions, and the terms are absolutely not interchangeable. If tests scores suggest that your son has a verbal IQ (that is, his ability to use and understand language) of 105, that's right in the middle of the average range. You say, "in all other areas his IQ is a 66." This might mean that his non-verbal skills, (that is, his eye-hand coordination and his visual perception) are extremely weak. This much of gap between verbal and performance IQs (the two major parts of an IQ test) suggests that at some point your son experienced a trauma to his brain that affected the centers that control non-verbal skills. So "by the numbers" he functions like someone with mental retardation in the visual perceptual areas. His verbal intelligence is clearly in the average range. If you blend these two scores together, you would get an overall IQ that would be in or close to the cut-off for mental retardation.
While it would be inappropriate to suggest a diagnosis from the information you've provided, this sounds like the profile of a student who has a significant non-verbal learning disability, or an impairment in the visual-perceptual-motor functions of the brain. For more about this condition, see www.nlda.org.