How are his legs? Any patella issues? Maybe he just likes soft surfaces.
You are training the "Down" just like I have for over 55 years.
I honestly had a dog who for 17 years refused to sit his rear on any surface in the "Sit" position. He just did not like his rear on the ground. It looked like he was sitting ... but he wasn't LOL! We lost a dog obedience contest because of it.
You are training the "Down" just like I have for over 55 years.