Is there such thing as "normal" anymore??!!
Honey, you just tell that sweet boy that his daddy is sick, and until he gets some help for his sickness, which could take a really long time, that it's not good to see him b/c the sickness makes him act differently than he should.
Being honest in an age appropriate way is the best thing you can do. It's great that you are there for him and making this very good decision to protect him from the harm it would cause him to be around that sort of toxic behavior.
Tight, tight hugs.