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If your son has not paid any income tax or national insurance for 15 years as he is unemployed, he should not be worried about that.
I presume he will be looking for work now that the inherited funds are running out. All is has to do is to register at JobCentre and explain his situation.
More information on how to contact Jobcentre Plus and jobseekers allowance can be found here
I hope this is helpful and answers your question.
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