The straightforward answer is that getting on that list as a girlfriend is going to be very difficult, and at most juvenile facilities it is not permitted at all under standard policy.
Juvenile detention centers deliberately limit outside contact to immediate family during placement. The reasoning is that the time inside is meant to be a reset, a period where the young person works on themselves without the distractions and social dynamics from their life before detention. Girlfriends, friends, and peers from before their detention are specifically the category of visitor that most facilities exclude, not because of anything personal, but because that outside peer contact is seen as counterproductive to the goals of the placement.
Going with your mom is a thoughtful idea, but it is unlikely to change the outcome. Your mom is not his family member, and the facility's concern is about who the visitor is, not who accompanies them.
That said, there are two things you can do that actually work. First, write him letters. Most juvenile facilities allow mail from outside the approved visitor list, and a letter from you is something real he can hold onto. Write as often as you want. Second, find out whether he is allowed to make outgoing phone calls and whether your number can be added to his approved call list. Phone policies at juvenile facilities vary more than visitation policies, and there is a real chance that calls are possible even when visits are not.
His parents or guardian would need to be the ones to advocate for any exceptions to the visitation policy. If that relationship is there, it is worth asking them to raise it with the facility director.