My kid is 5-year-old shy boy. He doesn't like to play with other kids!
Till Xmas day, his uncle gave my boy a present, colorful but huge blocks.
In the beginning, he always asked me to play with him, helping him building a big castle that he can hide inside. (He always likes doing this.)
Then I tried to tell him, you can invite your friend to play with you. You can work together to build a bigger castle.
Last weekend, he was courageous enough and asked two of his friends to come to our home and play.
I believe, really, good toys willl help kids develop well!