My son is almost three and his father has visitation but hasn't seen him in over 9 months. I heard that after a certain amount of time you can file for abandonment against the other parent. When can I file and how do I go about filing?

You can use the search feature on this site and find lots of other threads that will give you the answer. Basically you can't. So read around and if you still have more questions then please come back to this one and ask them.

And what do you expect to get out of this.

Not all states even have a abandoment statue, others it is merely a method of holding the person for non payment of child support.

So how long have you been divorced from the father. What is the child custody agreement, and how much is the child support order for. Is he still paying child support