This is one of the harder realities of juvenile facilities, and the answer is not what you are hoping to hear.
Juvenile detention centers operate under much stricter contact rules than adult jails or prisons. The policies are designed around the assumption that juveniles need to be insulated from outside influences while they are in placement, and visitation and phone access is typically limited to immediate family only. That means parents, siblings, and grandparents. A girlfriend, regardless of how serious the relationship is or how long you have been together, does not fall within that category under standard policy.
Without special permission from the facility director, visits and calls from a girlfriend are generally not going to be approved. That permission is not impossible to obtain but it is not common either, and it would require someone, likely his parents or guardian, to advocate on your behalf to the administration and make the case that contact with you is in his best interest. That is a high bar and facilities are cautious about granting exceptions.
The most realistic path forward is to stay connected through his family. If his parents or a sibling has regular contact with him, they can pass along messages and let him know you are thinking about him. It is not the same as hearing his voice, but it keeps the connection alive while he is in placement.
When he is released or transferred to a less restrictive setting, the contact rules may change. Focus on being there when that happens.