Hi Jan,
Don't see any born in Abercorn on the IGI, but noticed the following 2 in West Lothian:
William Boag, christened 19th December 1796, son of Thomas Boag & Margaret Black
William Boag, christened 15th July 1798, son of James Boag & Catharine Young
I had a wee look at the 1841 Census and it just states West Lothian as his place of birth - do you have a note of this particular Census? (It lists William age 40, ag. lab., wife Margaret age 25?, Mary age 13, Catherine age 8 and Elizabeth age 5 all living at Craigton, Abercorn)
Is Margaret's age a mistransription? I haven't look at any other Census years, but obviously having a 13 year old daughter would be 'pushing it' slightly!!

If Margaret's age is correct, then had he been previously married to someone else?
Anne