As mentioned, it starts from the first date he entered the UK on his spousal visa.
I see on your signature that it says you entered the UK on August 25th 2015... so that means his 12 months was up on August 25th 2016.
So, if he's still driving on his US licence, he is driving without a valid licence and without valid insurance...which means he needs to stop now, and only drive on a UK provisional licence, abiding by UK learner driver rules.