It's a drive from Beaverton, but Oxbow park would be a great place to start. Easy access to a number of proven holes. You also have the Wilson, but that requires a bit of scouting to find some nice holes.
You could also hire a guide to increase the odds of the boy landing a fish, and learn new water. The fist lesson he needs to learn is that you have to make a whole lot of casts to see a salmon on the bank.