I can tell you that MS in children is rare, and 'child' goes up to 18 years. My son in law was diagnosed with MS at 16 and even in a pretty big MS practice at the University of Minnesota, he was considered young for MS.
As far as symptoms, they are extremely variable, so you will really need to know about the individual child to be able to know what care he will need. Some people have frequent MS episodes, some are infrequent.
Some don't have any problems between episodes and don't need to do anything special (for example, my SIL has some balance issues that are not readily visible).
As the other poster mentioned, people with MS tend to have a hard time with the heat. But, that's not universal, so it's possible an individual may not have problems or would have problems only with heat combined with humidity. And, some are fine with the 80s, while others feel temperatures less than that.