Next-Generation Networks (NGNs), comprising for example, B4G and 5G cellular systems, will support Quality of Service (QoS) over a heterogeneous wired and wireless IP-based infrastructure. A relative model of service differentiation in DiffServ architecture is a scalable solution for delivering multimedia traffic. However, the dynamic nature of radio channels makes it difficult to achieve the target quality provisioning working separately at the IP and lower layers, as in the classical approach. This work describes an IP cross-layer scheduler able to support a Proportional Differentiation Model (PDM) for delay guarantees also over wireless. The key idea is to leverage feedbacks from the lower layers about the actual delays experienced by packets in order to tune at run-time the priority of the IP service classes in a closed-loop control. The objective is to implement a PDM on the whole at the network interface, as relevant for the end-users. A simulation analysis demonstrates the prominent improvements in reliability and robustness of the proposal with respect to both the classical and the open loop approaches. Considerations on the required functionality and possible deployment scenarios highlight the scalability and backward compatibility of the designed solution.