did you talk to your son whether he would want to live with you (and with new mother)and also you can give him better amenities and comfort. Is he willing to leave his grand parents just because they have not be able to provide him comforts. what is childs decision??
your ex is still single mother and needs to work to sustain life and give good education to son. she has been getting all the support needed from her parents.. what is wrong in taking parents responsibility. they too are looking after the child. just think they may be poor but still they have provided education to all the childrens including ur ex. Raising the child is also not easy task at the old age.
there are childrens who also study in empty school premises under the street lights. some childrens like to study where ever they feel comfortable. what if you had provided the separate table and chair to study. it would have been good gesture from your side to get his confidence.
it makes lot of difference if she calls him and is in contact everyday and visits only twice and you visit 4 times and no contact at all.