These are all great questions. It is true finding a good slp can be difficult, but once you find a good one it can be game changing. I'd listen to that podcast series I posted below. That particular series focuses on what stutterers, many of whom are SLPs themselves wished their therapists knew about stuttering. It can give you a good base. If the SLP has your kid read sounds on a sheet, practice breathing to reduce stuttering, or any other activities that reinforce to your son that stuttering = bad, fluency = good (which your son can't control), I'd start getting weary of whether or not therapy is helping or not. I'd also check in with your son how he feels about therapy.