Is anyone here familiar with Goldman-Fristoe Test of Articulation 3?

DS7 has been getting speech therapy all year. At first, the therapist was pretty sure he'd improve after just a few sessions, but he hasn't. So now they decided to do a full language assessment--partially because his speech isn't improving much and partially because we requested acceleration next year.

He took the GFTA-3, the EVT-2, and the PPVT-4. On the EVT and PPVT, he scored 135 and 139 respectively. But on the GFTA, his score was 45, or <.01 %-ile. Now, I know his articulation isn't great, but it doesn't seem *that* bad. His "sounds in error" are r to w, th to s, and sp to s. However, most people can understand him in context. They're setting up an IEP meeting. So... how valid is this test? And what should we be doing about it?