A very difficult situation if the court has accepted the plea and pronounced sentence. The plea language may even waive legally available appellate rights. Likely, he needs to consult a lawyer about post-conviction relief. The availability of relief is limited to very stringent standards such as actual innocence, lack of factual basis, etc. a very specialized area of practice.
I hate to hear these stories. There is always something you can do at the trial level. The arena he is in now is much more difficult.
Best Regards, Jeff
He needs to discuss this first with his defense attorney. Generally, he should have been advised of the nature of the plea and the consequences. But, unless the plea agreement reserved the right to appeal he probably waived rights to a direct appeal. If his defense attorney does not adequately explain now, he may have to discuss with new counsel whether there was ineffective assistance. An ineffective assistance claim is extremely hard to establish, but that may be his only recourse at this point.
This answer does not create an attorney-client relationship. It is advisable to consult with an attorney with full disclosure of relevant facts for a comprehensive leagl opinion.