Since there is refraction of the light in the water, so you should choose the lenses that are below than usual myopia 50 ~ 100 degrees. You should notice that it shouldn’t be higher than the usual degree, otherwise your son will be dizzy.
If your son has different degrees of two eyes, the degree of swimming goggles should also be different. Just select lenses that are 100 degrees below his optical glasses, if he doesn’t have astigmatism.
I have bought my pair last month. My prescriptions are 400 degree of left eye and 450 degree of right eye, and my eye doctor suggest me to choose the lenses that are of 300 and 350, just reduce 100 degree, and they work well.