There are specific time limits within which someone is allowed to withdraw/vacate their plea. There is no such thing as a "replea". In Florida, depending on whether he received a 25 yr mandatory or not. If mandatory he does the 25 yrs without gain time. If non mandatory, he can receive credit for up to 15% off the sentence. You need to contact a qualified criminal lawyer immediately to determine if there are any grounds and if there is still time to file a motion to vacate.
If your boyfriend signed a plea agreement then he is bound by that agreement unless he can assert that his lawyer was ineffective or the sentence is illegal. An illegal sentence would be when he is sentenced to a longer term than allowed by law (25 years for a 2nd degree felony, for example).
His lawyer being ineffective can happen when the lawyer gives legal advice that is fundamentally in error. For example, your boyfriend was advised to plea to the court and if the judge gave a harsh sentence the plea could be withdrawn.
These are rare but they do occur.
Generally speaking, when someone accepts a plea negotiation and then decides to try to get out of it, it is a simple case of buyer's remorse, and there is no way to get around it. Proving ineffective assistance of counsel in a plea bargain is often impossible unless the attorney admits his error. Illegal sentences are a bit easier and naturally more rare.
You would need a criminal defense attorney to actually look over the filings in your boyfriend's case to make a determination about it being illegal or wrongly advised.