If it has been less than 6 months since you sent your child to live in OR, then you probably would be able to get the case heard in WA.
We had an excellent attorney out of Bellingham named Ron Hardesty (located near the court house). You should really get as much documentation together and pay for a lawyer consulation to see what you can do now.
Depending on the circumstances, especially if your child has not lived with your aunt long, you probably have a decent chance of getting custody back. How soon and what you would have to do, I'm not sure.
Why are you doing parenting classes now? Did CPS request this, your aunt? Have you used drugs in the past? Could you pass a drug test now? These are all pertinent things your attorney would need to know.
You may have every right to take your son back at this moment, but I would definately talk to a lawyer first. You aunt will probobly just call CPS for help and then you are back to square one with CPS.
Good luck.