This is a civil case, not a criminal one. His 5th amendment protections are much weaker. If he says that his testimony may support criminal charges, then he is allowed to take the 5th. However, in a civil trial, the fact finder is allowed to draw a negative inference from that.
Having said that, none if this is relevent. He already testified during the State’s case, which is the only time he would need to invoke privilege. Since this is the defense case, they get to simply not call him.
Unless one of his co-defendants subpoenaed him, which is also not the case.