I am going to assume with - 6 you mean a measurement of - 6 Dioptre . If you are talking about the strength of the glasses he needs, the minus indicates that this eye is nearsighted, a positive value would indicate farsightedness. If that is a measurement of his eyesight, he is farsighted. A measurement of 0 indicates that the eye can adapt to both short and long distances without problems (though corrections may still be necessary, for example for astigmatism). Because of the definition of a Dioptre, there is no maximum/minimum number.
A dioptre (uk), or diopter (us), is a unit of measurement of the optical powerof a lens or curved mirror, which is equal to the reciprocal of the focal length measured inmetres (that is, 1/metres). It is thus a unit ofreciprocal length.
A measurement of -6 is certainly not nothing and will require corrective lenses. It is, however, far from being blind and can be corrected by wearing glasses or contact lenses. For nearsightedness, -6 is where high-degree myopia begins.
If you are talking about whether he could be considered legally blind , that is a definition that only applies to how much you can see with correction. Since myopia of -6 can be corrected well with glasses, there should be no risk of being defined as legally blind.