I doubt that you'll ever get cast iron "proof" that he changed his name. I can't find anything in the London Gazette where any Deed Poll would have been published. School records and the army records will be in his old name. Marriage record and after are in his new name. Where was he between the 1857 Indian Mutiny and his marriage? It's all just circumstantial evidence - which points to the conclusion that all of us have reached. If he never spoke about his parents how could his family have known anything about his father?

James Forbes appears to have died in the Sept quarter 1861 in Woburn. Did he leave a will? BLARS doesn't have one.

Good luck

David