My son didnt start speaking till he was 4. He started as an echopheli, so we had the school buy one that was used with pec pictures and voice. It worked great cause he learned the pecs and when pushed a sentence. Once he madtered one it was exchanged with another pec and different request. Plus we had a pec book at home with same pics. I had them taped to my fridge fridge for "drink "; cabinet for "cookie";ect. I also made a book for the car if i forgot to bring the one at home and another kept at my inlaws cause we went there all the time.
Keep pushing with the speech therapist on the benefts and how versatile the touch talk would be and how well it will work with pec system.
Ipads weren't around years ago but now schools use them all the time. Sometimes they say they don't want it to come home, that's a fight you need to decide if it is really worth it or you just buy your own for home. Keep fighting for what you think is right for your child