not sure if im reading this right but are you looking to pop the head back on to the neck post ball joint
if so i would immerse the head in boiling water for a couple of minutes and push it on to the post ball joint
or if your trying to push the ball joint back into the neck post ,ive not done it this way but i think ivanski did ,heat up the neck with either a hairdryer or boiling water ,a elastic band around the cape holding it to the body helps greatly lol ,and when softened push the ball joint peg in to the neck post ,hope this helps
tom k
the chest and back are made of, if not rubber its has rubber like properties and wont harden like plastic when reshaped ,it looks quite natural on mine and most of the pictures ive seen ,im wondering if yours could be warped or defective in some way ,any pictures
the only way i can think to remedy this would be to pad the chest and back cavity with that cotton wool like stuff they stuff cushions with ,ive not opened him up ,but feeling around i think if you opened the trunks at the seem between his legs you could push the stuffing in from there ,a bit like stuffing a turkey lol ,this should help to hold the back and chest cavity to fill out the parts that you refer to ,again i hope this helps good luck mate and let us know how it works out for you ,