Is it possible to call IRS to release the other's parent's W2 if he/she still not cooperating? Discovery is an expensive way to go if the tax information is available.
I cannot recall ever subpoenaing records from the IRS. It would likely be easier to subpoena employment records.
If you demand a copy of the ex' 2012 tax return and do not get it, file a request for orders compelling responses and the judge would most likely order them to hand it over.