Milestones for preterm infants

My baby boy is turning 4 months tomorrow. He was born 2 months preterm at 31 weeks gestation. His corrected gestational age is 2 months. How can I compare his basic milestones? like what age will he start to sit up on his own or at least by support? is there anyone out here with preterm babies? at what age did your babies achieved their basic milestones? this one is my first baby and well, you know how it is with your first ones...