You can find a lot of the MA Vital Records to the Year 1850 at http://ma-vitalrecords.org/ Unfortunately, the children's first names weren't recorded for this family. Looks like he might be the child born Mar. 1800?
Ipswich Source: Vital Records of Ipswich Massachusetts to the End of the Year 1849, The Essex Institute, Salem, MA, 1910
Ipswich Births
HARLOW
–––––, ch. John, Oct. ––, 1786. CR4
–––––, ch. John, Jan. 2, 1789. CR4
–––––, d. John, Apr. 27, 1791. CR4
–––––, s. John, June ––, 1793. CR4
–––––, ch. John, Sept. ––, 1795. CR4
–––––, ch. John, Aug. ––, 1797. CR4
–––––, ch. John, Mar. ––, 1800. CR4
–––––, ch. John, Mar. ––, 1802. CR4
–––––, s. John, Dec. 20, 1809. CR4
CR4 = Chebacco Parish Church (Chebacco later became Essex, MA)
Ipswich Marriages
HARLOW, John, and Eunice Lufkin, Jan. 10, 1786.
Essex Marriages
HARLOW, James, and Judith Roberts [of Gloucester, Mar. 1, 1828. int.].
Gloucester Marriages
HARLOW, James, of Essex, and Judith Roberts, int. Mar. 26, 1828. [Mar. 30. dup.]