Gail, there were several Edward Callow births within a couple of years either side of 1787.
I looked at public member trees on Ancestry and could find only one which suggests names for the parents of the Edward Callow who married Mary Prussia. This is "Callow-Tom Family Tree" and its compiler, Ron, is a direct descendant----but he hasn't used his tree for over a year.
Ron suggests that Mary Prussia's husband was Edward Smith Callow baptised 1 May 1787 Malew IOM to parents "William Callow Esq and Sarah Moore".
Copied from familysearch:
Name: Edward Callow
Event Type: Intended Marriage
Event Date: 10 Aug 1813
Event Place: , , Cheshire, England
Marital Status: Single
Spouse's Name: Mary Prusia
Spouse's Marital Status: Single
EC bachelor hatter
EC signed and MP made her x-mark.
Witness John Wood
Edward's fluent signature (unusual in 1813) suggests that he was from an educated family, so I decided to look at Frances's Manx Notebook summaries of old wills to see if William Callow or Sarah Callow nee Moore left wills, in the hope that there may be mention of a son Edward.
William and Sarah both died in 1791 leaving several young children:
Copied from Manx Notebook:
791-1 E w CALLOW Sarah dated 11 mar 1779;husb wm (hb c'town) exor - see wm
1791-1 E w CALLOW William d 11 jun 1791dated 3 jun 1791;high balliff c'town;niece bell_ann christian;bro-i-law thos howard;nephew thos christian;son george(land etc lezayre),wm(houses etc c'town);dau cath(her gfa george moore),margt;james kelly + thos howard guardians;wife sarah d july but wm omitted to have will proved;detailed inv + sale etc inc library
This was intriguing---- partly because there was no mention of Edward (and no burial of Edward Callow between 1787 and 1791), and partly because this promised to be an interesting will.
So I looked up the will reference (106402) and trawled through lots of images on familysearch, and eventually found the wills.
link to images of 1791 wills of Wm Callow and Sarah Callow nee Moore:
pages 368 to 411 of 772
I was hoping that Edward's name might have been there, but no mention of him. I wondered whether Wm Callow, who was probably unwell when he made his will a few days prior to his death, might have omitted the name of his 4 yr old child by mistake.
Have you any suggestion of names of parents for your Edward ?
I found an image of the parish register entry of the Edward Callow / Mary Prussia on familysearch ---- I don't think that Cheshire images have been viewable online for very long, so I'll attach it here in case you haven't seen it.