You can probably put DS5 in the "invisible but very real" disability category as well. I have a connective tissue disorder called Ehlers Danlos Syndrome (EDS). My kids have a 50% chance of inheriting it from me. DS5 seems to have it and will be seeing a geneticist in a few months. It is difficult to diagnose in kids before they are five. He's been in OT for the symptoms for over year. Writing or coloring for very long is painful, he has low tone, postural instability, hypermobility and problems with proprioception.
Reading Lori H.'s posts actually put me on the right track with what was going on.
EDS can manifest differently among family members. Since it is a collagen problem and collagen is found throughout the body, it can really look different in different people. While my son and I both have hypermobile joints and some skin issues, our secondary issues are different (at least for now) but all fall under the "things that can be wrong with you if you have EDS" umbrella.
While it is bad to have a rare genetic disorder, the good news is that if he does have it, there seems to be a supportive national foundation that has anticipated the needs of students with the disorder. Here is a nice guide they make available.
http://www.ednf.org/documents/EducatorsGuide%28r2011%29.pdf
