The Pilgrims found themselves three goals down at the break but did rally to cut the deficit through Holland's eighth goal of the season in the 59th minute.
Eastwood added a fourth and fifth to kill off any hopes of a sustained United comeback.
Boston (4-1-4-1): Jake Frestle (Nick Ivanov 46); Nathan Stevens (Finlay Armond 58), Jamie Gaukroger, Daniel Fisher, Owen Sears; Lewis Bontoft; Xander Crashley (Callum Harrison 66), Marshall Young, Mido Turki, Jenson Bark; Harrison Holland.