After saving the Minutemen from the Raiders at the Museum of Freedom, talk to Preston Garvey. You can join his cause and pursue a slew of Minutemen quests or you can continue your search for Shaun. Whenever you decide to look for your son, your journey will take you to Diamond City. It's south and a tad east of Concord, with a lot of locations in between, so be prepared for an eventful journey.
Once you reach Diamond City, you'll encounter a news reporter named Piper at the gates. She's having a disagreement with the mayor, which is why she's stuck outside. During her argument, hostiles will invade the area, which will bring out the city's enforcers. Now that the gate is open, you can enter Diamond City.
The quickest way to conclude the quest is to talk to Pastor Clements, who should be standing outside of his chapel. It's the first shack on the right upon entering the city. If you disclose that your son is missing, he will suggest you speak to Nick Valentine at Valentine's Detective Agency. He actually won't be there, but talking to his assistant will cause the quest to conclude.