I agree with everything said before me.


About the Spasms
That is just a natural reaction if you brush up against a python, as in they will retract their muscle or bend where you touched them.

Feeding
I agree that he should be fed rats at this point; they are more nutritious and will help him put on weight quicker. You should feed him a rat that is a little bit bigger than his girth once a week on a specific feeding day. If he is really underweight or scrawny, then you might even want to lower the time between feedings. Also, I recommend using frozen rats, if possible. A live animal could be additional stress on him.

Another thing that might help is some extra supplements (calcium and Vitamin powder) to help him heal.

Temperature/Humidity
I keep my room temperature at 80-85 degrees, which for you should be the cool spot, then the hot spot at around 90 degrees. This is very important for the snake to feed, and if it is too cold, the snake could get a respiratory infection.

Humidity should be at around 50%, which you can achieve by misting or adding a layer of a substrate, such as coco fiber or some jungle blend, under the aspen. If you were to do that, make sure it doesn't cause the aspen to become wet, which would lead to mold.

Aggression
Give him some time to heal. I know it is hard to do (I remember my first snake), but especially with this type of situation, it would be important to not handle him. Let him warm up.

Once he has recovered, you can start taming him. I have plenty of snakes that hiss at me at first, but after lots of handling they learn to love it. My oldest ball python is the biggest softie in the world.

Health Issues

Check him for respiratory infection and mites, both are high possibilities considering his past life.

Respiratory infection link:
http://www.anapsid.org/rti.html

Mite link:
http://www.anapsid.org/mites.html


By the way, a really good book to read is The Complete Ball Python by Kevin McCurley