In the next few years, a considerable innovation concerning the design of the future 5G mobile networks will be a concrete step towards enabling effective high throughput and low latency services. Software Defined Networking (SDN), Network Function Virtualization (NFV) and Self Organizing Network (SON) are considered the enabling technologies to achieve these goals. In this paper, assuming a Control-Data Separation Architecture (CDSA), we propose a flexible SDN/NFV-based SON testbed, for future 5G mobile networks. The main contribution of our work is to cover the need for a CDSA based testbed, enabling the investigation of the NG-SON capabilities for practical implementations. We implement two different testbed setups, a real one and a virtualized one, both based on the FlexRAN and OpenAir-Interface software tools. First, we implement a specific case study, i.e., the RAN entities activation/deactivation procedures. Next, we carry out time measurements, concerning the aforementioned procedures, in order to prove proper testbed functioning. Finally, we validate the C-SON and D-SON capabilities of our testbed, considering the features of the results.