I would advise him to go into them and channel his inner Mitt Romney, just sound and act as reasonable and bland as possible. The only people he can add to his voter base would be centrists who are put off by his brash personality. So to get them on side he needs to downplay their reasons to dislike him, and at the same time he could calmly, reasonably state what good things he has done and will do in the future, contrasting that with the potential harm of the opponent’s stated plans.
I don’t think skipping the debates would cost him much either. Basically there is almost no part of his declared base of support that he can lose. His main threat is vote fraud rather than the popularity of any opponent. He should invest a lot of effort in fighting vote fraud ahead of the 2020 election.