Statute of Limitations - Paternity/Child Support

Are there any time limits on requesting child support in Washington Sate?

Here’s the story:

Spouse had an affair approximately 20 years ago. Mistress got pregnant 3 years later. She was also fooling around with 3 other guys as the time of the pregnancy, so there is a 25% chance is or isn’t his. He asked her for a paternity test when she became pregnant, but she refused.

She finds us out of the blue a few weeks ago, asking for him to see “their” son. Granted, we were never “hiding”. She got married 14 years ago, when the son was 3, and now he is 17.

If the child is his, would he be responsible for any support? The child was raised by his stepfather for 13 years. Also, she supposedly didn’t list anyone as the boy’s father on the birth certificate.

Help!
Thanks!