Being President tends to age people rapidly. His diet has served him well for the most part all this time, I wouldn't mess with it until he's out of office.
I disagree that his diet has served him well previously. A high fat diet, even if it doesn't make one overweight (though, he is overweight), is bad for you all around.
If I were his Dr, I'd get him on a healthy diet immediately. Even if he doesn't have heart disease, he'll live a longer and healthier life with a good diet and some exercise.
If he were 90 I wouldn't mess with his diet, but as young as he is, he would benefit from getting healthy.