the feathers he lost seem like flight feathers if they are below his wings and closest to his tail then they are flight feathers and yes you should probally clip the other side as well cuz that is how they maintain balance ... but id not advise clipping them yourself ... enless u really know what ur doing .. u could hit a blood feather or clip too close ... most pet stores do it for free.
just to be safe id take him into an AVIAN vet first thing in the morning .... keets .. like most wild animals... will over compensate when ill or hurt ... survival of the fittest kinda thing ... so if he is hurt even a little bit he will try his best to cover it up.
and they really are so so fragile teeny tiny bones and stuff ... although very flexible it doesnt take much to do harm ... id take him in just to be sure. dont want him to be suffering in secret
and i can imagine how you feel ... it happens they are extremly fast sometimes and they move way too quick ... just gota be super careful
im certain you didnt intend to sit on him ... it was an accident and it does happen .... just be sure he is ok