my niece and nephew, their dad (mybrother-in-law), his mother and her father ALL walk on their tiptoes. My niece in particular walks on her toes so pronouncedly that she is an absolute natural at gymnastics. She sprints along the bar on her tiptoes as you are meant to when all the other kids have to learn how to do it.
I would suggest that their is quite a strong suggestion in this case that tip toes is another inherited trait as brown eyes/long fingers and skin colour are.
i'm sure they could be a symptom of ADHD, but it also could just be a conincidence. Does your daughter walk on her toes? Also, ADHD is not under the autistic spectrum disorder I don't think.
to put your mind at rest, see the health visitor and talk to his teachers with a view of seeing the ed pysch.
good luck, and try not to worry. In my experience as a teacher and parent of 5 most boys show signs or autism and ADHD at some point in their lives. Try upping his physical activity significantly and see if this makes a difference and don't allow him to get bored.