He should be able to obtain his certificate of disposition from the superior court which sentenced him and any correction if he was sentenced in error below the minimum that could be imposed. But there must be a record of that or DOC should not be able to hold him. If he completed his federal sentence that should have no bearing unless there was concurrent time involved with the bit that he is serving now. Where is his attorney at sentencing? He should contact Prisoners' Legal Services.
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If he was determined to be a predicate felon, it is possible that his sentence was 1.5 to 3 years. You can go to the court where he received his sentence and check what the actual sentence was.
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