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Because he pled guilty, he has little recourse. He voluntarily gave up most of his rights by deciding to plead guilty. I'm assuming at this time it is too late to file an appeal. At this point, he can try to file a post-conviction petition alleging ineffective assistance of counsel that would get him a new trial.
Just so you know, however, counsel is not ineffective because the information about the victim's sexual history was not brought up. That is absolutely NOT admissible in court during a rape trial. The issue is whether he had sex with an underage girl. Period. She could have slept with 500 other guys, and it doesn't have any relevance to whether this individual had prohibited sexual contact with a minor. So if that is what you are basing your belief that his conviction should be overturned on, he is not going to prevail on that argument.
But if he has other grounds, I would go the post-conviction petition route.