It seems that your son's left eye has been irretrievably damaged and eye ball is shrinking as it is not even producing fluid to maintain the eye shape. This is called Phthisis bulbi and at this stage he can not even perceive light through this left eye.
Although psychologically, this may be painful for you as well as your son but in the interest of future health and comfort of this eye as well as right eye, my advice would be to have the left eye removed and then have the artificial eye fitted. This will avoid not only future problems but will also look cosmetically better than at present. Your local eye specialist should be able to advise you on this and also where to get the best place to get an artificial eye in Mumbai.
The eye drops he is having is just to keep the eye as comfortable as it can be. I think he will be far more comfortable and happier in the long run without it as it has no future and is a potential liability.