Is a mother sinful if she prevents her ex-husband from taking their son, or prevents him from seeing him online and asks him to communicate with her parents to follow up on the child's condition?

The father has the right to visit and see his child, and this is a right agreed upon by jurists. As for taking the child to his home (at the age of two), there is no religious obligation to permit it because it is for the father's benefit, not the child's. However, this falls under good treatment and benevolence, unless there is a confirmed harm to the child, such as the fear of abduction. Regarding the father's online communication to talk to his son, you can refer him to your father to speak with him, and you are not obligated to communicate with him directly since he is a non-mahram. Be careful not to prevent him from connecting with his son, and ensure that one of your mahrams is present during the call, and adhere to the Shar'i hijab if you need to appear on camera.