Hi
you should file for divorce on the basis of cruelty and desertion
Do you meet your son, how often you meet him and your wife.
Ask for the Permanent custody and for now the access to the child in the same Petition.
Since the child was living with her there is a chance custody can be denied to you
Immediately after the matter begins in the court file interim application for access right and seek for order to visit your son as you want.
Even if the custody ordered to her you should ask for access regular, visits and 15 days continuous stay with you when child has holidays .
You can also try and suggest for a Joint Parenting depending on the circumstances you three live,and the nature of the dispute with your wife.Many couple opt for joint parenting where both have equal right an liability and access, in which an agreement is signed regarding the schedule and terms of parenting and child can stay with both equal time.Then there is no custodian parent an access parent. This can happen if you bioth can reach to anunderstanding