When strangers wants to talk/play with my 21mths toddler, he ether cries or look warily at them. how to ensure he grows up with good social skills?

Passing Stage!: Stranger anxiety is very common at this age; it is probably the rule rather than the exception. This is a passing stage and your toddler will eventually become more interested in exploring the world while developing independence. It is important to keep exposing your toodler to social situations rather than sheltering them from interaction. Stranger anxiety little to do with future social skills.

His reaction may : be a from a slow-to-warm-up or shy temperament plus stranger anxiety. To reassure yourself that his social communication & reciprocal social interaction are on target, use the developmental screens & M-CHAT on firstsigns.org. He should parallel play with peers during playdates & demonstrate pretend play. Have adults greet him at his eye level from a few feet away instead of looming over him.