Since this is an error in metabolism, he may have severe cognitive, physical, and developmental issues. The severity of the associated sequelae really depend on his disease progression. At this point, without knowing the child, it would be very difficult to recommend any specific approach to treatment. However, to prepare, I would recommend getting some records from the primary provider. His medical records will give you a more clear idea about his issues, especially if he has severe cognitive impairment. Also, it will be important to know the onset of the disease and how quickly complications have developed. Once you meet with the child and the family, you may want to focus on what areas of his development they are finding the most challenging. It is quite possible that the child may present with overwhelming issues. Focusing on small goals, one at a time may be the best approach. Also, I would recommend discussing the family's perception of his disease progression. If they are realistic about the disease and prognosis, it may make goal setting more realistic. He clearly has a communication delay. Also, he may have some impulse control issues. In addition, if he's not appropriate for therapy, then he's not appropriate for therapy. Sometimes that does happen. Obviously, I'm an NP, not a SLP, however, I work closely with ST/SLP and have worked with several special needs children. I wish I could be more specific. However, again, it all really depends on the child. This child may have different needs than other children with the same syndrome. I realize that this answer may not be as helpful or specific as you would like, so, if you would like a more specific answer, I can opt out of the question. Just let me know. I will completely understand.
Also, if you have follow up questions or concerns, please let me know.