Welcome to Flyertalk!

Since you'll be connecting from IAD to BOS, you will not be riding the "Moon Buggy." Instead, you'll follow the signs for "Transfer" and be processed through Immigrations and Customs in the facility on the lower level of the midfield terminal. You'll then go up the escalators or elevator, and proceed to the gate for your flight to BOS.

As a family traveling together, your husband should join you and your son in the line for US Citizens. My sister-in-law does not have a US Passport, but she always goes through the US Citizen line with my brother when she visits the US.