For the second time inside a week, United suffered defeat to a side earning their first victory of the National League campaign - this time with Moors leaving the Jakemans Community Stadium with all three points.
Two well-taken goals from Jacob Wakeling, a recent signing from Peterborough United, sandwiched Hiwula's reply to seal victory for the Moors, under the guidance of caretaker manager James Quinn.
The Pilgrims, attacking the North Stand first, gave possession away cheaply in the 15th minute and WAKELING crashed home a beauty, on the angle, off the underside of Rhys Lovett's crossbar.
Lovett's outstretched leg denied Oscar Rutherford a second - and United capitalised with Manni Norkett and Michael Kelly combining well down the left for the latter to whip in a cross which was slammed home by HIWULA.
Second half chances were at a premium, although the Pilgrims did have to withstand a succession of Solihull set-pieces and long throws.
Their resolve was undone when they thought they should have had a foul on Hill deep inside Solihull's half, but referee George Laflin and assistant Alan Hull waved play on, with Jamey Osborne playing the pass of the season through for WAKELING to run on and angle a low effort beyond Lovett.
Sheffield United loanee Owen Hampson made his debut as a late substitute, scuffing one effort wide, while Hiwula crashed an effort off target following Lenell John-Lewis' flick - as the Pilgrims ran out of time to force a second equaliser.
Boston (3-4-1-2): Rhys Lovett; Connor Teale, Zak Mills (Jacob Scott 66), Alex Lankshear; Ben Grist (Owen Hampson 80), Jordan Richards, Greg Sloggett, Michael Kelly; Dylan Hill; Jordy Hiwula, Manni Norkett (Lenell John-Lewis 66). Subs (not used): Rocco Coyle, Arjan Raikhy, Luca Alonzi, Bobby Jones (gk).
Solihull (4-2-3-1): Laurie Walker; Oscar Rutherford, Alex Whitmore, Brad Nicholson, Cameron Green; Jamey Osborne (Oliver Tipton 86), James Clarke; Bradley Stevenson (Emmanuel Sonupe 72), Ben Worman (Callum McFarlane 72), Jacob Wakeling (Szhem Whyte-Hall 86); Dan Creaney. Subs (not used): Sam Bowen, Louis Jackson, Rory Brown (gk).
Referee: George Laflin.
Attendance: 2,012 (86 visitors).
PIC: Chris Bray.