If you have substantial evidence that this has been his consistent way of working throughout his time in secondary education then you might be able to use a file note illustration of persistent difficulties that would put him at a significant disadvantage in comparison with other students without such a disability when taking the same exams.
Page 4 of JCQ regs will give you further information to assess whether this would apply to your young man, and, if in doubt, ask Nick Lait at JCQ for clarification and include his advice in your evidence folder.
Hope this helps 😊