He came home from Iraq in 2007, he suffered his accident and memory difficulties in 2009. It therefore follows he had 2 years when he would have known he had the weapon, even if he subsequently forgot.
The plea of mitigation concerning the loss of memory is tenuous, for if he did not declare the weapon between 2007-2009, why would he have suddenly declared it later? On that basis I can understand why he was advised to plead guilty and did so - and why the judge was dismissive of the plea for mitigation.