Sometimes the way to address problems with state agencies is to start at the top.
Your son should write a letter to the Governor's office explaining the problem and asking for help. The letter should be short (no more than a page, single space) outlining the injury and what L & I is doing. Ask the Governor to look into this. (Gregoire won't see the letter - it'll be handled by one of her staff members).
Here's what will most likely happen. The Governor's office will send a copy of the letter to whomever is currently the head of L&I (I've lost track) and ask for a response within 30 days. The head of L&I will send the letter down the chain of command, find out what's going on from L&I's perspective, and write back to the Governor's office, who will then write to you. Or, you will hear from L&I.
This may or may not get Mark anywhere, but it is worth trying because (1) it shines a light on what is going on and (2) it's free to try.
As a person with lifelong back problems myself, I have an idea of what Mark is going through. I can tell you that medical massage and chiropractic treatment help me tremendously, but I know exactly what you mean about the pain pills clouding his mind. Still the letter should come from Mark, even if you have to ghostwrite it for him.
There ARE attorneys out there who may be able to help him. If you let me know what county you're in, I may have further ideas.
But he should look for Worker's Compensation specialists, even if he has to go to Tacoma, Seattle, Spokane, or Vancouver to find them (by phone should work).
Please let me know if I can be of further help. Sometimes it's effective to write to your state senator and/or representatives, but I like starting at the top - that's why I suggested the Gov's office.