The money on his books is not the issue. The issue is the contact policy at juvenile facilities, and it works very differently from adult jails and prisons.
Juvenile correctional facilities restrict communication to immediate family members only. That means parents, siblings, and grandparents. Girlfriends are not included in that category regardless of how serious the relationship is or how long you have been together. It is not personal and it is not about you specifically. It is a blanket policy that applies to every juvenile in the facility because the system is designed to limit outside peer influences during the rehabilitation process.
The money you put on his books can still be used by him for commissary purchases, which means it is helping him even if you cannot hear his voice. That is not nothing.
The most realistic path to staying connected right now runs through his family. If you have any kind of relationship with his parents or a sibling, reach out to them. They can tell you how he is doing, pass along that you are thinking about him, and let him know you put money on his account. It is an indirect connection but it keeps the thread alive.
Depending on how long he is at Topeka Youth Center, the contact rules may relax somewhat over time with good behavior and program participation, or they may remain strict for the duration. His case manager or the facility chaplain would know whether any exceptions are ever made for long-term relationships, though the baseline answer is almost always no for juvenile facilities.
Stay in touch with his family. That is your best option right now.