Do not deposit or cash the check yourself. It is not yours to negotiate, even with good intentions, and doing so could create a legal problem you do not want.
The right process is to contact his counselor at the Cherokee County Adult Detention Center and explain the situation. Tell them his paycheck arrived and you want to get it deposited into his commissary or phone account. The facility will instruct you to send the unsigned check directly to them. Once it arrives, they will have your boyfriend endorse it, and the funds will be deposited into his account.
Do not write anything on the check yourself before sending it. Keep it exactly as it arrived and send it to the facility address the counselor gives you, along with your boyfriend's full name and inmate ID number so it gets routed to the right account.
This is a routine situation that jails handle regularly. Calling the counselor directly is the fastest way to get the specific instructions for that facility.