G.B.
cord blood stem cells have been shown to help with cerebral palsy.
Low dose naltrexone has shown a lot of promise for rigidity as seen is things like parkinsons. I would definitely give that a try if I were you. Do a search on it online, you will see it really helps with muscle problems.
Also, you should consider if the child has low potassium/high cellular sodium- this is a red flag issue when there are heart issues like tachardia.
Tachardia is a symptom of high cellular sodium. Cells should be full of potassium, not sodium. Ask your docotr if you can get a prescription for potassium. make sure you monitor it with blood tests.