So this YP hasn’t received updated advice since year 7 and is now in year 10?
Does he have an EHCP?
Has he been in different schools or the same for this period?
Why do you think having him reassessed will be a waste of money/time? My understanding is once your goven a diagnosis unless there is evidence to question the original thought then a diagnosis of however long ago made stands and any good EP in accordance with regulations would adopt those already identified diagnoses at the forefront of any readable report.
Having an EP re-evaluate needs is critical especially is one has an EHCP and also upon transitioning phases. The provision of professional evidence is just that, you obtain said reports because teachers are unable to carry out certain assessments and an update provides a full, clear and current picture of current presentations and needs, not what they were some 3 /+ years ago.
There are other factors to academic ability, levels of Social engagement maybe hindering in areas, life skills, everyday adaptive behaviour in particular in light of an ASC profile.