If he’s into any particular activity enroll him in a program. Baseball, soccer, art, music etc. Somewhere he’s likely to interact with kids he doesn’t know. My daughter is in her 1st year at Madison, most of her friends went to Denny, and she’s not yet making new friends. At least not any that come over. The teachers and other parents typically say that the new friend making happens moreso after 6th grade.
Maybe check with Lafayette, Schmitz and Alki elementaries and see if they have any suggestions for programs or activities. Those will be the kids he’ll mainly be going to school with next year. Good luck.