You don't need to do nuffin. It makes no difference to you. Here's why...
In any rear-end collision, the vehicle that did the rear-ending is 100% responsible for hitting the vehicle in front. Every time, no exceptions... except for the one this guy doesn't know.
The one (and only) exception would be if his vehicle was completely stopped and pushed / thrown into yours. When that happens, his vehicle stops being a vehicle and becomes a projectile. But he only gets off the hook if he can prove his truck was at a dead stop, not rolling in any way, and he will be asked to prove it. And he can't prove it, because the damage will tell the truth.
If his truck (fully stopped) was actually hit from behind hard enough to be thrown into your vehicle, he will have massive damage to his rear end. His entire cargo box will be destroyed, and the vehicle that hit him won't be able to leave the scene without a tow truck. And the insurance companies know it, because that's what they do for a living.
The guy who hit you heard somewhere that he can claim he was hit from behind and pushed into your vehicle, but he doesn't know it only works for fully stopped vehicles. He probably won't even say he was fully stopped, he'll say he was driving a safe distance behind you. He might even try to claim that the road was slippery, that your brake lights weren't working, that you stopped suddenly for no reason, that his windshield was fogged up, or all of the above. Because he thinks that will get him off the hook for your damage. And he'll be wrong, because none of that stuff matters. When his insurance company sees no (or very little) damage to the rear of his truck, they'll accept 100% responsibility on his behalf. So make sure you have a repair estimate ready, because they'll want to see it before they approve your repairs. And they will.
But what if this guy still insists it wasn't his fault, you ask? What if he gets himself a lawyer and threatens to sue his insurance company (and you, and whoever else) if they don't change their minds? Nothing happens, nothing at all. Insurance companies are totally allowed to accept liability (fault) on behalf of their customers, and it says so right in the contract. Their decision is final.