Today at my husbands support group meeting, a speech therapist and her partner were there talking about" Interactive Metronome." It works with brain injured patients to reset their brain through timing and rhythm. You can look it up here. interactivemetronome.com. It seems to have a lot of success and is covered by Medicare and most other insurance for all ages.
My daughter is also a speech therapist. She works in a different category, early intervention, with preschoolers. She said she has heard of it and that there are continuing education courses, seminars and other info. for practitioners. Speech therapists have Masters Degrees and are pretty savvy. It sounds legit.
I would suggest you look it up as it sounded quite interesting and we are going to get his doctors recommendation and referral to try it. From what I understand, after some assessment, it is 4 mornings a week for 30 minutes for about 6-8 weeks and one can come in later for a "tune up" when needed..
PD and Speech Therapy
Difficulty with speech 
Possible improvement of speech in Parkinsons patients with the encouragement and implementation of a daily walking routine. 
Eye Guide MC
World-class Speech Therapy - Offered weekdays - No Charge
