If you have no court-ordered custody judgment, then you and your child's mother have equal custody rights to your child. Most custody judgments contain a provision that prohibits either parent from moving the child out of state without the other's permission. If you think she's going to do this, you should consult with an attorney immediately, and file a petition to establish custody and parenting time rights. It will be a lot easier to do this before she moves, and to prevent her from taking him away, than it would be to get her to bring him back, once she's already left.
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Mr. Bodzin has given you good advice. I'd like to just address your comment to his answer regarding not being able to afford an attorney.
Whether or not you can afford an attorney, it's important that you file your petition ASAP (before she moves). If there was never any sort of judgment, you need to get this in front of the court. If you cannot pay an attorney to do this for you, go done to the courthouse (if the child really lives with you 50/50, then this could be the circuit court in the county you live in or that the mother lives in) and file a couple of documents. The first is the petition for custody, parenting time, and child support that was already explained by Mr. Bodzin. The second is a motion for something called a "status quo" order which explains where the child has lived for the past three months and asks that this schedule remain the same until further order of the court. Please note, you need to file the petition before you can file for the status quo, but it's okay to do both on the same day.
Obviously it would be in your best interests to have an attorney help you with these documents, but it is also important to act fast. One thing I suggest to my own clients is that they ask relatives for help with a retainer. Also, many attorneys can take credit card payments. If neither of these is an option for you, you can always file on your own and perhaps seek an attorney to represent you at trial. You will also need to make sure the mother gets personally served (someone other than you or your attorney has to give them to her in person) with these documents very soon after filing. You can arrange for the Sheriff's Dept. to do this while you're at the courthouse.
You should also ask at the court to waive (and if they won't do this, to defer) your filing fees.
There are several resources for low income clients that you should check out, such as:
The Oregon State Bar's Modest Means Program (reduced hourly fees of $60, $80, or $100, depending on your income): http://www.osbar.org/public/ris/ris.html#referral and (503) 684-3763, or toll-free in Oregon at (800) 452-7636.
Legal Aid Services of Oregon (I'm giving you the info for their Clackamas County Office, but if you live in the Northern part of Wilsonville, you might need to contact their Washington County office instead): http://www.lawhelp.org/program/1173/index.cfm#Clackamas and (503) 655-2518 or (800) 228-6958.
St. Andrew Legal Clinic (again, if you live in Washington County, contact that office instead): http://www.salcgroup.org/our-locations and (503) 557-9800.
My responses to posts on AVVO are not legal advice, nor do they create an attorney-client relationship. In order to provide true (and reliable) legal advice, an attorney must be able to ask questions of the person seeking legal advice and to thus gather the appropriate information. In order for an attorney-client relationship to exist, you and I both have to agree the the terms of such an agreement.