For a 2-3 year old that is a huge amount. It's likely he has pathological high myopia. Extremely important for him to be under the care of a pediatric ophthalmologist. Will need glasses, eventually contact lens or refractive surgery and the eye has a higher chance of having problems like macular degeneration or retinal detachment. No way to tell you what his vision will be or IF he will develop these problems. Was he premature? Or low birthweight? Or had to have oxygen at birth?