Child custody

Sir, My ex' husband has taken my son;s custody.My son now 5 years old.I had visitors right only & I used to take my child on weekends.But suddenly He moved to out station (from Kolkata to Chennai), I was taking care of my child fully 7 days in a week,though legally I supposed to not but as he is my child I didn't think twice, I took whole responsibilities.But now I got transferred to another city, I had to join there anyway. Currently my son has been taking cared by his grandmother,she is 65 years old lady.My son & his grandmother stay together, My ex- husband used to visit them 4 times in a year. My son's education getting affected, his health mentally & Physically getting affected, lacking confidence, from schools getting report of poor performance.Even most importantly my son doesn't want to stay with her ,because of her cruel behavior. Sir all the time a 5 years old baby can not cope up with a 65 years old lady.My son is not getting father& mother's touch,the vibe, the nourishment , which the kid must need in this age, the growing period. I want to take his custody anyway, I can not see my child to suffer like this.Please give me a fruitful suggestion.