Walaikum O Assalam,
My son had the same issue, his numbers were -5 but Alhamdolillah that we kept reciting Quranic verses & Duas on him & my mother used to put Zamzam in his eyes frequently; now his numbers have dropped to -1.50 Masha'Allah. His age is currently 14 & during the last 2 years, I have performed mild Hijamah on his neck & head and the Sunnah treatment also helped.
Kindly read the article Verses & Duaas for improving vision >> https://goo.gl/jAFUEr
Furthermore, it is my personal experience that taking saffron with hot milk improved my vision & focusing ability, now I can read smaller fonts without glasses. I took camel’s milk but any kind of good quality / fresh milk can be used.