You are very wise not to trust online trees. They may be right, but very often they are not. Never trust anything you find online unless it's an image of an original document.
Unfortunately people tend to copy what they find online without checking for primary evidence, so one unverifed suggestion can quickly proliferate into multiple trees perpetuating the same assumption/error.
Have you looked at the sources quoted on the various trees you have found and worked out which tree(s) have reliable-looking sources - that is, a source that is
not one of the other trees or an index or transcription on Ancestry, FamilySearch, MyHeritage, Geni, Wikitree etc? You could try contacting the author of that tree and asking what their evidence is for the parentage of John Forrester?
I see that John F and Helen Shaw had at least 9 children, and that the eldest son was William and the second daughter was Elizabeth. If the Forresters followed the naming tradition, this could point to John's father being William and his mother Elizabeth.
However he named his third son Alexander and he had no (recorded) daughter Helen, which is not in accordance with the naming tradition, so you can't be absolutely certain they were following it. Also, in the apparent absence of a marriage record, there could have been one or more unrecorded children before William.
Do the children's baptism records say what John's occupation was, or where he lived, or who the witnesses to the baptisms were? The latter can often provide a clue to earlier generations.
There could be mentions of John Forrester in the Kilmadock Kirk Session minutes. He might have been an elder, or he might have borrowed money from the Kirk Session - I was able to work out the parentage of one of my ancestors from the receipts in the kirk accounts (in another parish, not in Kilmadock). The kirk session records are in the National Records of Scotland. They have been digitised and are to be made available on the Scotland's People web site, but I don't know when. It could be a long job to search through them for all mentions of John Forrester! You could consider hiring a professional searcher to do so on your behalf, of course.
It might be useful to find out who owned Ashentree Farm? In 1855 the proprietor was Miss Clementina Graham of Coldoch. There will be a record in the Registers of Sasines of who owned it before her. It's also just possible that some estate records have survived that might contain relevant information.
See
http://www.stirlingarchives.scot/2016/01/18/coldoch-blair-drummond/However all of these approches really require you, or someone on your behalf, spending time in archives in Scotland, as any information is not going to be online.
