S.H.
Well, he's only 21 months old,
At this age, they do "parallel play"... not interactive play yet.
And they don't know/understand all social nuances yet.
Their emotions, are not even developed yet, nor, fully developed yet. At this age.
So, the world is still one big puzzling thing to them.
And, they are reactionary at this age.
They cannot necessarily be real 'articulate' verbally or with body language yet... and have the verbal skills to say "stop it" to others or offer problem solving, yet. They do not yet, know how to 'problem solve' on their own and they do not have, 'coping-skills' yet at this age. Socially.
He will be fine.
Phases.
Age phases.
Just do not, make it a big deal. Or then he may get hang-ups about himself. That is worse.
all the best,
Susan