I have sole custody of my son. Can my ex force me to move closer to him to make his visitations easier?

It sounds as if your ex lives in CO. The laws of each state may be different from those of other states. You may need to review your facts with an attorney who is licensed in CO to see what CO laws are.

If you have court orders regarding your son issued by CO courts, CO courts may still have the powers to decide issues regarding your son.

If there are no court orders and your child has just moved away from CO within the last 6 months, CO courts may still have to powers to decide issues regarding your son. With the filing of an appropriate petition, CO courts may order that your son be returned to CO for the courts to decide what to do with him.

If there are now no court orders regarding the son and you have lived in WA for 6 months or longer, WA courts likely are the ones with the power to decide issues regarding your son. I know of no WA law that would force one parent to move closer to another parent so that the other parent can more easily see his child.

The father may be referring to filing a motion/petition with the appropriate court asking the court to declare the father the custodial parent (the one with whom the child lives most of the time). Given that the father "has not seen [your] son since just before his 2nd birthday (He is 9 now)", it is unlikely that any court would sudden change the child's living arrangement unless the court deems living with you is unsafe for the child.

If you are served with any court documents, you should promptly check what you need to do with those documents.

You should review your specific facts with your attorney to see what your legal options are.