That is a very wise decision. His advisors should tell him to stick to it.
He has absolutely nothing to gain from debating, and a lot to lose. He must stay in control of the circumstances, and any debate format involves a loss of control and a necessity to share the mic.
His hard-core supporters tend to watch Fox News, which always presents the president in the best light. It doesn’t matter what kind of scandals are exposed on the other networks, because his base doesn’t watch them. But if the debate is on CNN, his followers will be watching, and that gives CNN a chance to tell them the truth. Moderators will be asking and following up on tough questions, and analysts will be correcting his false statements with facts.
No matter what other decision he makes, he definitely should not agree to a debate format with a live audience. His schtick doesn’t work without his hand-picked, low-information audiences. The statements that work at his rallies might well be laughed at by a randomly-selected audience, much as they were at the U.N. And I don’t think he understands how to modify his routine to fit the circumstances. He seemed genuinely surprised when they laughed at him in the general assembly.