Thanks for using the forum. I am happy to address your questions, and my answer will be based on the information you provided here. Please make sure you recognize that this forum is for educational purposes only, and it does not substitute for a formal office visit with your doctor.
Without the ability to examine your son and obtain a history, I can not give you a diagnosis for him. The causes of hypotonia and thin myelin are multiple, an entire book could be written about those to conditions alone or in combination. Without further details, it is impossible to narrow the list down. However, I will try to provide you with some general information.
Our nervous system consists of the central nervous system (brain and spinal cord) and the peripheral nervous system (nerves in our arms and legs that come out of the spinal cord). We also include muscles within the realm of neurologic problems. Myelin is the substance that coats our nerves, both in the central nervous system and peripheral nervous system. Demyelination, or loss of the myelin sheath, can occur around the brain and/or in the peripheral nerves.
Hypotonia, or low muscle tone, can also result from problems in either part of the nervous system or muscles.
The causes of demeylination and hypotonia could include genetic problems, metabolic problems, mitochondrial problems, and toxic problems (exposure to toxins during pregnancy or after birth). The central nervous system demyelination disorders as a group can be called leukodystrophies, and include metachromatic leukodystrophy, Pelizaeus-Merzbacher Disease, Alexander's disease and Canavan's disease, Krabbe's disease, and so on. The leukodystrophies are often associated with hypotonia early on, and in some cases this is later replaced by increased tone, or spasticity. If the demyelination only involves the peripheral nerves, then the general term would be demyelinating neuropathy, and this would include such conditions as congenital hypomyelination neuropathy, the hereditary sensory-motor neuropathies (charcot-marie-tooth), and others.
Each type of condition can be identified based on a combination of the history, physical examination, and lab testing.
These are but a few examples. On the surface, your son might fit the description for any of these conditions, but evaluation by an experienced physician will identify some additional features that may not be obvious to you that will help direct the diagnosis. The most important advice I can give you is that in cases such as your son's, in which a diagnosis may be difficult to arrive to, it is important to be evaluated at a center that has the capabilities and expertise to diagnose your son. Evaluation at a tertiary care hospital (a large academic center with a good reputation that has specialists in various fields) is important. I feel your son would benefit most form evaluation by both a pediatric neurologist, a metabolic specialist, and maybe a genetic specialist.
Thank you for using the forum. I hope you find this information useful, good luck.
Thank you for your answer to my question. We are seeing a pediatrck neurologist at the University Hospital. The just did more testing so hopfully that will lead to something.