Tummy time for preemies?

My son is almost two months old and he was in the 95th percentile for development compared to other babies that have just been born. So basically his skills are developing and he's doing great but I was wondering about tummy time. When do you start doing tummy time with a preemie? When I hold him sitting up and he's against my chest he'll try to left his head but I kind of restrict it from moving too much since he doesn't have all of that head and neck control and support yet obviously. How do you know if they're ready? I don't want him to hurt himself on accident.