In WA, while a private person may person a citizen's arrest under limited circumstances, private persons generally do not have the power to arrest, charge, or prosecute anyone.
Merely impregnating a willing person above the age of sexual consent generally would not be a reason for the police to arrest the impregnator. In WA, the age of sexual consent is generally 16. Unless you are alleging that the sexual activity is not consensual, the police likely is not going to get involved. Other reasons why the police may get involved even though you are over 16 include if you two are related within a certain degree (such as he is your uncle) or if he is your teacher or someone with a supervisory role over you.
The police (and the court) is not an enforcer of personal relationship (although some persons do use the police and the court for revenge).
In WA, both legal parents have duties to their child. Those duties include financial support for the dependent child. Once the child is born, you can ask the court to set child support for the child.
Your more immediate concerns may include your immigration status. Given your age, you likely are in high school. Most international students attending high schools or lower schools are authorized to be in the US for only a year. Your authorized stay in the US may be ending.
If the child is born in the US, the child would be born a US citizen. If the child is born outside the US and the father is a US citizen, additional steps must be taken for the child to be a US citizen.
Until the child is at least 21-year-old, the child may not help your immigration status.
You can apply for medical assistance for the unborn child regardless of your immigration status.
There are many issues you must be aware of. Frankly, you are a little bit young. You likely will need help on many different issues. If the adults around you are not yet aware of your pregnancy, you likely should tell them now. Filling out all the forms to qualify for various services do take time.
This is not an immigration question. I'll re-classify so that you can talk to a family attorney.
FORMER IMMIGRATION LAW PROFESSOR -- LEGAL DISCLAIMER: This answer is offered for informational purposes only. It does not constitute an attorney-client relationship.
Why would you want to arrest him? You may have chosen to date the wrong guy, but that doesn't mean he committed any crimes. As mentioned above, this is a family law, not criminal law, issue.