I would suggest that he didn't work for Carron. There were lots of iron foundries nearer at hand.
The Falkirk Ironworks was established in 1819 and taken over by Kennard in 1848. It was the second largest foundry in Scotland. Abbots ironworks and foundry opened beside the canal, east of Bainsford in 1856. Grahamston Ironworks opened in 1868. Castlelaurie Ironworks opened at Bainsford in 1875. And there were others.
Since people tended to live quite close to their work, it's worth looking at a contemporary map to see which ironworks were nearest to his home.
I see Andrew is with his parents (father a slater) at Kirkliston in 1881. Still in Kirkliston in 1891. In 1901 he is at 22 Balmoral Place, Galashiels, described as clerk to an iron foundry.