Overweight fiancé

My fiancé is a very sweet man who wears his heart on his sleeve. He is overweight and was when I met him. However since we've been dating he has gained 30 pounds! He is a large football player type build of 6'3" and 350 pounds. He tries to diet but then impulsively eats. How do I get him to lose weight for our wedding??? It's getting a bit embarrassing that he's gotten so heavy.