illegal sentence Sentencing, Plea Bargains

my boyfriend has been in prison for eighteen years, he did a plea bargin for 2nd degree attempted murder. after reviewing his records i noticed that they sentenced him with attempted 2nd degree murder with the use of a firearm which gave him a 3 year min mandatory which he can not get gain time for. he has filed a writ of mandamus. the courts told him he was right but since it was done in an untimely fashion they had to deny him. he even has asked for another hearing.. but denied. he is doing an appeal now but my question is this, is there something else we can file so that he will get his gain time?
please help us figure this out.
thank you

 

maybe i put this in wrong to where you can't understand what i'm saying..ok ...
what if someone is in prison in florida, serving a sentence beyond the statutory maximum punishment for his crime, and a crime he was never formally charged with, that he can prove by the face of the record that it is illegal, by which method does he use to prove this to the courts? he has filed a writ of mandamus, and the court response was even tho he is right it was done in an untimely fashion. he asked for a rehearing which was denied, now he is in the process of doing an appeal...i feel that we are not going about this right, he has been in prison for 18years please, any help would be appreciated.