The Census records state he was born in Peterhead. The reason may have been that his parents were wed in Peterhead on 9 Jul 1826---eight months after Andrew's christening---and I believe Mary THOM was from Peterhead and that she died there in 1861.

IGI records have: Andrew SHUAN s/o Andrew SHUAN/Mary THOM chr 24 Nov 1825 Longside Source C112185 Dates 1782 - 1855 Call #0993340 Parish registers for Longside, 1621-1854 Church of Scotland. Parish Church of Longside (Aberdeenshire)Andrew SHUAN, son of Andr. SHEWAN, chr 03 May 1730 Peterhead Source: C112322 Call # 0993353 Parish registers for Peterhead, 1664-1854 Church of Scotland. Parish Church of Peterhead (Aberdeenshire)
Printout # 6901243
In 1841, fifteen-year-old Andrew SHEWAN, a seaman apprentice was living on Errol Street with his father Andrew, a seaman and younger brother, Alexander also a seaman apprentice. [Source 232, district 0, folio 8, pg 15]
In 1851, Andrew SHEWAN, a shipmaster, age 30 born in Peterhead, was residing at St. Peter's Street, Peterhead with his wife Jane, age 28 from Aberdeen and their one-year-old son Andrew [folio 178, pg 26, #129],
Fifty-five year old Andrew SHEWAN, from Peterhead, was the ship's master of the vessel "Sweet Home" in North Leith, Edinburgh at the time in the 1881 enumeration. Crew members included John Robertson, 55, the Master; Alexr. Cordiner, a Mate; John Robertson an Able Bodied Seaman, and John Florence a Seaman --- all were from Peterhead.
Five years later, in the record of marriage for his son Andrew's marriage to Isabella Cordiner STEPHAN, indicates Andrew SHEWAN [father of the groom] was a ship captain.
In 1891 he was enumerated with his daughter Jessie SHEWAN in Peterhead, Aberdeen.
Andrew's death record indicates that he was a merchant shipmaster, the widower of Mary GREGORY and living at 42 Maiden Street. He died at age 66 from apoplexy from which he had suffered 5 years andfrom softening of the brain from which he had suffered for 3 years. Both of his parents---Andrew SHEWAN, merchant seaman, and Mary ms THOM---predeceased him. [Source GRO 1893 Deaths in the district of Peterhead in the county of Aberdeen pg 57, record 169]

The children of Andrew and Mary Gregory included
Mary b abt 1854 married John MAVOR 1880 in Peterhead [GRO 1880 Marriages 232/01 0045 page 23, Peterhead, Aberdeen]
Williamina Gregory born Peterhead married John PERKINS in London, England
Jessie b 11 Aug 1858 Peterhead died 1858 [ 1858 Births in the Burgh of Peterhead, county of Aberdeen, pg 64, line 191]
Janet b 19 Apr 1860 Peterhead / d 15 Sep 1933 Aberdeen m George YOUNG 1906 Peterhead then to Joseph HOOD [1860 Births in Burgh of Peterhead, pg 21, line 91 / 1933 Deaths in Northern district of Aberdeen, Aberdeen---Janet HOOD married 1st to George YOUNG, fisherman; 2nd to Joseph HOOD, .]
Andrew b 28 Oct 1861 Peterhead m Isabella (Bella) Cordiner STEPHEN 25 Mar 1886 Boddam, Aberdeen [1901--Andrew, age 39 was a lace maker worker living with his 37-year-old wife Isabella and children at 19 Prestage Buildings, Poplar, Tower Hamlets, All Saints Parish, London]
Elizabeth b 29 Nov 1863 Peterhead m 1886 Middlesex, England to Charles Scott WATSON [believe she died in Los Angeles, California]
James Stewart b 24 Jun 1865 Peterhead
John b 6 Jan 1867 Peterhead
Helen Campbell SHEWAN b 30 Jan 1869 / d 1890 St Nicholas, ABD
Amelia Ferlina SHEWAN b 17 Dec 1871 Peterhead, m: Robert Lennox CLARK 1904 Vancouver, BC, Canada [1901, Farlie Shewan, age 29, unwed, a housemaid, was residing in All Saints Parish, London, England with her brother Andrew]