LL
Congratulations on the award.
It's unusual but there are no rules preventing such a term.
Without anything else happening, then your son's award will end on 23/07/29, however, he should be contacted for a review in advance of that, anything up to a year.
in practical terms, there is no difference between a new claim and a review, the same criteria are used and any assessment, if one is required, will be the same, the only actual difference are the form and whether those assessing him refer to the documentation from his previous claim, which they often don't.
Gordon
Nothing on this board constitutes legal advice - always consult a professional about specific problems