Wow my so baby is still a baby

My SO son is four and I think he's a little slow I'm just noticing how he seem to not be acting like other 4 year olds its probably because he's not in preschool or around other kids. Kindergarten is coming he's gon be the smallest in class. I wonder if he was a preemy or was my SO a premature baby I'll ask later. Maybe his mom is slow lol....