18 month old toddlers don't typically play together, let alone play actual games together. At best they play alongside each other, seeking each other's proximity without much interaction. If the other kids you see are playing games, they are much older than your child.
It sounds like you can't find the middle ground here. Either you just keep your child home, depriving him of social experiences, or you keep pushing him to play with others. That is not something you can expect him to be developmentally ready for.
Your toddler's character isn't unique, your expectations are.
Take your boy outside to experience the world. If he only stares at other kids, let him. His observations of social interaction will help him when he's old enough to join in. If he wants to explore on his own, let him. If he wants to stay by your feet, let him. Let him see you chat with another mum. Even only watching and listening will teach him vital social skills.