Based on the fluctuation-dissipation theorem, this work is concerned with the dynamic form factor of Thomson scattering for ion-collisional plasmas. The main effort is to calculate the ion susceptibility by analytically solving the linearized ion Fokker-Planck equation through the Chang-Callen 13-moment approach. Comparison to other theories shows that this theory can provide an accurate prediction of the position and width for ion-acoustic peaks and a proper description of the entropy wave with some underestimation of its damping. The theory may be useful for explaining experimental data and making measurement more precise.