That sounds so weird! How is he getting into the car? Does he show any fear at all then? The whining and trembling - that is during the ride or just when it's time to get out?

And what about the return ride home - does he get out of the car on his own when you return home?

I would think the picking on the other dog that he does is a dominance thing, trying to establish order. 4 months still seems very new to me. There are lots of threads on issues like that. My personal solution has been to have just one dog .