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Unless he entered a conditional plea, reserving his right to appeal some specific legal issue, he cannot appeal from this plea/sentence. Things like whether he was under the influence, Miranda violations, etc., raise factual issues regading which an appeal is possible only if they are litigated in either a pretrial motion or a trial.
It is extraordinarily unlikely that any appeal would now be successful. A petition for a writ of habeas corpus has a slightly higher chance of success, but still not good unless there is a significant legal issue related to the plea itself.
These issues should have been addressed prior to his plea, which does raise the question of whether he had an attorney and if so whether that attorney adequately represented him with respect to the plea. That is probably the only issue which has any real chance of success.