My question involves collection proceedings in the State of: WA

The back story:
Almost 7 years my wife and our son were involved in a car accident. My wife and I paid all of the medical bills before settling her claim against the other persons insurance company. We were under the impression that all of our sons bills had been taken care of either by PIP or by us paying them directly. We still haven't settled our sons claim with the other persons insurance company, because we wanted to make sure there weren't any long lasting medical issues. My son did have a lawyer representing him, but doesn't currently.

The current situation:
The other day we received a notice addressed to our son from a collection agency stating that there was an outstanding bill that they intend to collect on. This bill includes about $2000 in charges and another $1500 in interest (12% from RCW 19.52.010). To the best of our knowledge we never received any bills from the company in question, and we never remember making any payments to them either. Going back through the paper work from the lawyer I did find a list of visits from this business and some chart notes but nothing else.

I believe this debt may be valid, however I have a problem with the 12% interest they are attempting to charge. Like I stated before we paid all of the bills we received ourselves, and had this business sent us correspondence we would have likely paid it also.

My Questions:
1. I plan on requesting validation of this debt from the collection agency should this request come from my son or from me as his parent?

2. From what I have found the statute of limitations in Washington would be 6 years. Doing some reverse math on the numbers they sent they are charging interest for about 6.5 years. The last appointment from the list I found in the lawyer file seems to be about 6.5 years ago. Is it correct to use the last visit date for the start of the statute of limitations and assume that the statute of limitations has passed?

3. If I am correct in assuming that statute of limitations has passed should I state that in the validation request, or should I wait till I receive the validation response?

4. Any other information/help on this matter would be appreciated.

Thank you,
Thomas