Trump cannot decide to be president. God help us, he is the president.
Nor can he decide by an act of will to exercise the office of president in a mature or responsible way. He lacks even the minimum qualifications for the office he holds. He demonstrates no awareness of his inadequacies, no willingness to address them, no capacity to remedy them. He brings to the office a stunning combination of arrogance/ignorance, corruption/dishonesty, incompetence, intellectual laziness, bigotry, and emotional/mental instability. 
Trump cannot decide to be president; however, he can decide not to be: he can resign, and for the love of God and of the country, he should.
Trump cannot be forced to resign, but he can be removed by either of two constitutional processes.
Of course, in any of these scenarios we end up with either President Pence or Acting President Pence (depending on the avenue by which the Trump presidency ends.)
What a mess our country is in! What a tragedy for us and for the world!