Early intervention is the cure. Not many grand parents can take care of the children. so now, one need to be with the child always. he is just 4 year old. If proper therapies are given and the child is taken care with full dedication. I can assure you that he will come out of the mild to moderate autism definetly down the line.
Do not expect things to happen overnight. Gradually u will notice the changes.
If there is an option to hire a special educator. please do that. Keep on talking to him. Definetly he will improve verbally.
Set goals. Like for example. I have set a goal that i need to teach red color identification for my son in 20 days. everyday i used to show all the red color items to generalize. like that he started identifying red color. so here the duration of the goal is 20 days.
in the same manner u also set goals , to what to teach him. consult child psychiatrist , therapists. They will guide you accordingly.
Patience is the key. Trust your actions. Definetly your son will shine brighter. Good luck. Dont worry.
My Kid , a boy aged 5 years 9 month is going to Tulipians Primary School near Gariahat. It is good school having special educators and emphasis on special need child. I also heard about Akshar School near Kalighat being a good schoold for special need child.