I have several children on my caseload with suspected developmental apraxia of speech. I really like Easy Does it for Apraxia. It has physical cues for all sounds and simple art activities that kids seem to find motivating. It also provides you a good progression of skills... I typically make the program into a notebook that can be practiced at home too, so there is a lot of repetition. Also, have you considered some kind of augmentative communication? I find that my most severe children will more willingly imitate the speech output on a device, providing lots of repetitions throughout the day.
Thank you for sharing!