I answered you other post :-)
It doesn't sound like Sep. Anxiety, but more like he is tired...but not tired enough to get him self back to sleep. My LO does a 35min (exactly) in the car, i don't know why but if she is going to do a short nap at home it is 45min - weird i know. Anyway, as long as you are still putting him back to his crib awake and you think of the rocking as a temporary measure...then i don't think it is a step backward.
I would really look at extending his A time - try walking (but don't let him sleep) for the extra time, it is just the right amount of stimulation w/o making them overtired. Or walk around the garden...
Hope this helps.