We report the observation of well-resolved A and B excitonic resonances in the reflectivity spectra of a wedged GaN/AlGaN quantum well (QW) with 5% Al content in the barriers for different well thicknesses. For the thicker well cases, the energy splitting between the A and B excitons is larger than the one found for bulk GaN. However, the A-B exciton energy splitting decreases with decreasing well thickness, accordingly to the delocalization of the electron and hole wave functions in very thin QWs, as supported by theoretical simulations. Moreover, we used time-resolved photoluminescence measurements to investigate the recombination dynamics of the two excitonic states demonstrating the existence of a thermodynamic equilibrium between the two populations at a sample temperature of 10 K.