We investigate a microscopic model based on the pair approximation, for the dynamical properties of the photo-induced high-spin state in spin-crossover solids at low temperature. The model uses the Ising-like hamiltonian and combines long- and short-range interactions. The stochastic treatment of the latter provides a set of two coupled differential equations for the macroscopic short- and long-range order parameters, which reproduce successfully the main features of the experimental relaxation curves.