I agree with what Attorney Harlan said about DOC procedures, especially if your son is in a state prison. I am not familiar with the federal prison system, which may or may not have similar procedures. In the meantime, you should also be concerned about the the "federally equal to sexual rape" statement. I don't understand the qualifier "federally", as sexual assault is sexual assault whether under the federal criminal code or Washington's criminal code. However, if your son is being investigated for sexual assault of an inmate, staff or a corrections officer, this could lead to the filing of serious charges.
This is a tough question because DOC has it own rules and regulations once a person has been convicted and sentenced to prison. Technically, he was sentenced to 10 years and the good time he earned may be given or taken away based upon discretionary decisions made by DOC. I'd encourage you to hire a lawyer handling this unique niche. He will be entitled to some form of due process, e.g. a hearing, before his good time is taken away. If possible, he should be represented by a competent lawyer at that hearing.