I can understand the helpness you are feeling as a father. I was in the same boat ( I still think I am in same boat).
All suggestions and pointers I got said -- father seldom gets custody and if child is living with mother -- forget about it. I was working with one motive that lets put the effort - rest leave it with God.
My son is currently 6 year 4 months old. I am stationed out of Metro. The child is with mother for last 3 years 3 months.
The grounds I took in my application was health issues when the child is with mother, his improper education( he is attending a school which is not recoginezed by state or central education board, keeping child absent from school), legal non compliance and a chances of better all round development when child is going to be with me. I have given documetary evidences for each of these.
I know that above may not be at the level of details you need. Hope you can understand the reason why I I need to keep the informaiton at this level