Dear, Proving desertion is not that easy, think from the perspective of your wife's lawyer also, You think he will accept whatever you submit to the court ? You will be able to prove it but it will take time in contested divorce cases, no matter how strong your evidences are.
You should emphasize that you were anyways maintaining your wife and child when they were with you, and its your wife's voluntary decision to seperate the child from you without any valid grounds, a child needs both parents and not just the mother. Also mention that you reserve right to file custody of your child.
Good that you understand your responsibility as a father but don't get over emotional , do your calculations properly and commit for a decent amount for your child. Ask your wife to submit expenses she incurs for maintaining the child. Your willingness to spend more for your child will be considered as your capacity to pay for your wife's maintenance, mind you !! and this will be counterproductive in case the court goes ahead and grants interim maintenance for your wife as this stage. In my experience courts are women friendly and hence the damages to husband are more since the merits of the case are considered after the husbands back bone is broken on account of paying huge maintenance to non deserving wives.
(My comments are under assumption that your wife has deserted you)