Growth parameters need to be plotted on growth chart. Parental heights need to be taken in account. Systemic examination required for your son.

What was his birth weight and how much preterm was he? What is his present weight? There are particular growth charts for both weight and height. If u can send his immunisation card, I think both his weight and height will be written there during each vaccination visit. Then we can check if he is growing normally for age or not. If it is shown as rising trend, don't worry as he was preterm.

He is near to normal. It also depends on what is the Corrected age ,according to Gestation age at birth. We have  to  do serial  measurements six monthly to  monitor growth velocity. Try to get plotted on growth chart during physical consultation.