If you have a parenting plan and the mother is not complying with the court order, why have you not go back to court to ask the court to enforce the order?
Consent of the legal parents make an adoption go quicker and likely less expensive.
However, consent is not absolutely required. If the court finds the parent to be unfit, the court will terminate the parent's parental rights and allow the adoption to go through if that is in the best interest of the child.
What you should do is review your specific facts with your attorney to find out your legal options.
I am an adoption lawyer, not a custody lawyer, so I will limit my answer to the adoption issue in your question.
In a stepparent adoption, the rights of the biological parent (you) need to be terminated before the court will grant the adoption. In Washington there are two ways your parental rights could be terminated - voluntarily and involuntarily:
Voluntarily: A parent's rights are terminated voluntarily when the parent signs a consent to adoption, and after a waiting period of at least 48 hours, the court accepts the consent and terminates the parent's rights. (This, by the way, can only happen if there is someone else willing to adopt the child and accept parental responsibilities. Even if both parents agree, one parent cannot simply be relieved of parental responsibilities without there being an adoption.)
Involuntary: The stepfather can ask the court to terminate your parental rights without your consent. In order to do this he needs to be able to prove by "clear, cogent, and convincing" evidence (a very high standard) "that it is in the best interest of the child to terminate the relationship and that the parent has failed to perform parental duties under circumstances showing a substantial lack of regard for his or her parental obligations and is withholding consent to adoption contrary to the best interest of the child." RCW 26.33.120
There needs to be a hearing on the stepfather's petition to terminate your parental rights, and you need to receive notice of it. If the child's mother and stepfather know where you are, they are supposed to have you personally served with this notice. If someone is trying to serve you with notice you shouldn't avoid them - you want to receive the notice so you can respond. If the child's mother and stepfather can convince the court that it is not possible to serve you personally with notice, the court can authorize them to give you notice by mail to your last known address and by publishing notice to you in a legal newspaper. Again, if they send you notice, it is important for you to read and respond to it.
Because of what the child's mother and stepfather will need to prove if there is a hearing (and because it is the right thing to do) it is important that you consistently do everything you are able to do to fulfill your parental obligations. Those obligations go beyond just paying child support, and include letting your son know that you love him, providing him with guidance, etc. If there is a reason you haven't been able to visit your son, then do whatever you can to call him, write him, send him gifts, etc. Also, it is very important to make sure that you are keeping current with your child support obligations.
Finally, not showing up for the hearing after you have been notified of it (either by personal service, or by mail and publication) is, all by itself, grounds to terminate your parental rights, so it is very important to show up for the hearing.
The stakes at a hearing seeking to terminate your parental rights are extremely high, so it is a very good idea to be represented by an attorney. It is possible that the court will appoint an attorney to represent you if you are not able to hire one, so if you are in that situation it would be a good idea to ask the court to do that.
Some biological parents decide to negotiate an "Open Adoption Agreement" so that they can consent to a stepparent adoption, be relieved of future child support obligations, and still preserve an enforceable right to some level of communication and/or contact with the child - but every situation is different and I don't know if that would be an option for you.