You might try having the private SLP talk professional-to-professional to the school SLP, so they can more effectively coordinate services (e.g., to use consistent cueing language with him regarding articulation, prosody, etc.). You will, of course, need to share the history of speech issues and therapeutic strategies with the private SLP at the outset of therapy, so she understands what some of your goals are in coordination of care, and has some context on possible responses from the school-based SLP, who may be a bit defensive about being contacted by a private apraxia specialist.
Then you can let the school SLP continue to work on social skills, while the apraxia specialist works on articulation. Speech/language stays on the IEP...and you're still out a lot of money, but at least the speech services are probably net positive.
