Ohhh...please don't put him in the truck bed..first off..the dog can't grip anything, as most vehicle beds are metal. Think about this..what if you had to slam on the brakes?? you know where Rocko would go?? out the side..off the back...
And the thoughts above are so true..he might see something "interesting" ( someone on a bike..another animal... ) and just leap..maybe right into traffic.
If you must take him.. ( know Ashley goes with me evrywhere that allows dogs too ) please get him a plastic kennel and line it so he has a bit of security, and bungee cord it down.. ( sorry..I heard about someone losing their dog because they got in a wreck and the kennel flew out, and got hit by another car.)
This is just my opinion, but I would rather have you come HOME to a safe little guy like Rocko, then to have to face the numerous dangers of a dog not restrained INSIDE a vehilce. Maybe it will make your trips when its just you and your boy a whole lot more special for both of you!!