DS2 is 10 & recently diagnosed with inattentive ADHD & autism suspected. We saw paediatrician in late July who diagnosed ADHD on the spot & he has medication (Xaggitin).
When we saw paediatrician she remarked he is very skinny & no muscle. She weighed him & he is underweight. He has always been skinny like his dad, who is 6'3" & 11 stone, at age 44, having recently gone up a clothes size.
Anyway, paediatrician referred him for blood tests. All came back clear except creatine kinase, which is slightly elevated- level of 406, she said normal is 40-320. She said cause for concern is usually thousands, but due to his build & muscle tone too, she wants GOSH to check him out to make sure all is ok. She said if he was sturdy she would not worry.
I have been googling & am absolutely petrifed! I keep coming up with Muscular Dystrophy & limited life span 
Does anybody please have any knowledge or experience they can share? I can't stop thinking about it. Is there anything I can do?
For info, he eats very well & always has done. He has a good off switch & will stop eating when full. He has put on approx 2 kilos since July, bringing him close to 3rd centile weight & not technically underweight. He can swim & ride a bike, surprising everyone, but he is not strong in sports. Although PE teacher says coordination is improving & also listening skills & teamwork.