Spasticity is usually caused by central nervous system injuries, such as brain injuries or spinal cord injuries. Spasticity is a little different from stiffness.
As your child with CP is becoming a teenager, there are a couple of things you may want to think about.
Let me tell you about multimodal communication. We, as typical communicators, all use many different strategies. We use speech, gestures, facial expressions, technology, and no-tech solutions. As communicators, we all size up the situation based upon our communication partners, the context, the environment and then we choose to use the most effective communication strategy.