If he moved to Newcastle in 1834, where were James b 1830 and Thomas b 1833 born? Where and when did he marry? And what information do you have that points to him being from Forfar/Kirriemuir?
James and Thomas were born in Edinburgh, but Alexander and Mary Ann were married in Newcastle 3 Oct 1834 at all Saints Church, Newcastle. I have not been able to find a baptism for James but Thomas and Charles were baptised 1 July 1836 at All Saints, with the year of birth for Thomas given as 1833.
The census for 1841 & 1851 Alexander gives his birth place as Forfar Scotland as were James and Thomas.
I did wonder if he had been a member of the Forfar Militia which had been based in Newcastle sometime in the 1820s. but a lot of those records have been lost/destroyed.