The Entity

The movie begins as single mother Carla Moran (Barbara Hershey) is violently raped in her home by an invisible assailant. A subsequent episode of poltergeist activity causes her to flee with her children to the home of her friend Cindy (Margaret Blye).
They return to Carla's home and the following day, Carla is nearly killed when her car mysteriously goes out of control in traffic. Urged by Cindy to see a psychiatrist, Carla meets with Dr. Sneiderman (Ron Silver) and tentatively agrees to undergo therapy. A subsequent attack in her bathroom leaves bite marks and bruises, which Carla shows to Dr. Sneiderman, who believes she has inflicted them on herself (despite the marks showing up in places impossible for her to reach). We learn that Carla suffered a variety of traumas in her childhood and adolescence, including sexual and physical abuse, teenage pregnancy and the violent death of her first husband. Dr. Sneiderman believes her apparent paranormal experiences are delusions resulting from her past psychological trauma.
Carla is attacked again, this time in front of her children. Her son tries to intervene but he is hit by electrical discharges and his wrist is broken. Dr. Sneiderman urges her to commit herself to a psychiatric hospital for observation, but she refuses.
After Cindy witnesses an attack, the two discuss possible supernatural causes. While visiting a local bookstore, Carla happens to meet two parapsychologists, whom she convinces to visit her home. Initially skeptical, they witness several paranormal events and agree to study the home. During their study, Dr. Sneiderman arrives and confronts Carla, trying to convince her that the manifestation is in her mind, but she dismisses him. Reassured that her case is being taken seriously, Carla begins to relax. Her boyfriend Jerry (Alex Rocco) visits and she suffers a particularly disturbing attack, which he witnesses. Hearing the commotion, Carlaâs son enters the room and believes that Jerry is harming her, prompting him to attack Jerry. Later at the hospital, Jerry is so troubled by the experience that he ends their relationship.
Desperate for a solution to her problem, Carla agrees to participate in an elaborate experiment carried out by the parapsychologists. A full mock-up of her home is created to lure the entity into a trap. Liquid helium will be used to freeze the entity, once inside. Before the experiment can begin, Dr. Sneiderman unsuccessfully tries to convince Carla to leave.
The entity arrives but unexpectedly takes control of the liquid helium jets and uses them against Carla. She defiantly stands up to it, stating that it can never have her. Dr. Sneiderman rushes in and saves her. As they look back, they see the entity frozen for a brief period into a very large mass of ice. It eventually breaks free and vanishes, but Dr. Sneiderman realizes that Carla was telling the truth the whole time. Carla returns to her house the next day. The front door slams by itself and she is greeted by a demonic voice which says, "Welcome home, cunt". She calmly opens the door, exits the house, gets in a car with her family and leaves.
A closing disclaimer verifies that Carla and her family have moved to Texas. Carla still experiences attacks from the entity, although they have lessened in frequency and severity.

Question: Who attacks Jerry at the hospital?
Answer: Carla's son