My son got lead poisoning an from the bad walls paint now t efect his brian to can I sue the landlord?

Generally the answer is yes you can sue the landlord. There are a number of factors that must be established. The cases are not simple. Most importantly you must establish that your son's lead  poisoning came from something in the landlord's apartment, and that your sons injury is long-lasting or permanent. Often liability  insurance policies have exclusions for lead,  but not always. It is worth exploring the case with a lawyer who handles these matters.