How to make child more sociable?

He could feel neglected now that there’s a younger sibling. Try to give him a role in caring for your younger child, so he feels involved and not neglected? Also see if your oldest child can play with him? He may not like playing with other kids yet, but going for play dates or play group may help (note: sanitise his hands frequently, because you don’t want him falling sick). Also, if you can have someone help with your youngest child sometimes, set aside some time every day with your son, and make it quality time, so he knows you love him.