Low muscle tone is a condition (not a disease) where the muscle’s readiness for action is somewhat slower and lower than would be expected for a specific age. It often causes children to be more “floppy” than would be expected. If severe it could lead to developmental delay in the young infant and baby.
As your son is 6 years old , I assume that it has not been troublesome until now. Children are often diagnosed during their first year of school as low tone around the shoulder area will cause them to slump, have a bad pencil grip and tire easily when doing writing exercises.
Your child should have an assessment with an Occupational therapist and/or Physiotherapist who will work out a program tailor made to his needs. With the right intervention he should be able to partake and excel at any activity and schoolwork.