There are three possible answer (one of which was already addressed in the comments.
- Cerebro is shielded, and any mutant inside Cerebro is safe from Charles' attack.
- Jason is a mutant with psychic powers, much like Charles. Not only that but he is (artificially) stronger than Charles, as he forces him to execute the attack. If Jason is capable of this, it's possible that he is capable of withstanding Charles' attack.
- Similar to point 2, Jason may be able to resist Charles' attack because of something that made him resilient during the process that made him the (freaky) psychic that he is in the movie. Much of Jason's power (and creepiness) comes from the fact that his own father experimented on him to turn him into a weapon. Who knows what they did to him.
Edit From the comments below, and the phrasing of the question, I thought of a 4th option.
When Professor does this, all mutants starts to scream and squirm, but Jason doesn't.
Jason is not normal, even by mutant standards. He does not talk, he does not even seem "human" (behaviorally). He is a psychic weapon more than anything else.
- Just because Jason wasn't seen screaming, does not mean he doesn't experience pain.
Maybe Jason did experience the pain, but he simply didn't show it externally as he is incapable of "normal" behavior. There is also no proof that Jason is even still capable of experiencing pain in general. The extent of the experiments that have been done with Jason could very well have rendered him either incapable of feeling pain, or given him the ability to ignore pain as an overriding emotional response. He was focused on forcing Charles, after all. And he possibly does not have a will to live, and therefore doesn't panic at the thought of dying.