A 70year old man is going through dialysis but he is refusing treatment and has to be sedated in order to get dialysis. What will this do to his health?

Consent? If he is refusing dialysis, it would typically (in a consenting patient) to sedate him in order to give him dialysis.
Hopefully None. Professionals in this setting are almost always highly trained and unlikely to take untoward risks. The use of sedative medications here presupposes adequate kidney function (to eliminate sedative byproducts) and the ability of these health professionals to "support" any reduction of blood pressure or cardiac output that may happen transiently.
If he is . Refusing treatment, but is of sound mind, you have an ethical dilemma. A family conference may be appropriate.
Ethical dilemma. Sounds like we have an ethical dilemma. If he has the capacity to make medical decisions and wishes to refuse dialysis it is inappropriate to sedate him for the purpose of gaining his consent. On the other hand if he does not have capacity to make medical decisions his surrogate should be acting in his best interest. Based on your story it seems like you should have an ethics consultation.
Consent. Who signs consent for this patient. If he is mentally competent he has the right to refuse. He can not be treated without consent. Dialysis will help him to avoid encephalopathy and hyperkalemia caused by renal failure.