I have suggested that he might have other reasons: he is perhaps well ahead in internal polling and sees no reason to risk a debate. I agree he might be negotiating this way.
As of now, with the polling the way it is and the state of play therefore adverse to Trump, I disagree with the decision, if final, to boycott the debate.
By the way good news Fox just came up with 49/48 Trump in Arizona.
You don’t like the President’s reaction because you credit the polls. If polls were accurate and honest, President Hillary Clinton would be running for reelection right now. Better to trust your own instincts about the state of the race than the polls.