I need some wisdom...

As the father of an adult son,
I know it is hard to take advice from someone who's diaper I once changed.

So my 2 cents.

Acknowledge first that he is the father and you are the son.
Tell him about your concern for your brother.
(I also am an older father with young children,
so I'd guess your Dad and I probably share similar anxiety about mortality and fatherhood)

Offer to help him with the changes he needs to make.
Be patient and be willing to follow his timeline for change
as long as it's in the right direction.

Your Dad is lucky to have a caring son like you.

I have the same problem.  My father is 63, morbidly obese, diabetic, and just all around not mobile.  He is an ex marine who used to be in great shape, and just decided that retirement means sitting on your A@@ all day.  

Now, I have had to go back to the old saying "You can lead a horse to water, but you can't make them drink."  I have learned there is nothing I can do to change his mind, but be a leader by example for him. I will cook them healthy meals that taste great, and it has led to a few changes.  He sees he doesn't have to add excess amounts of salt or sugar to things for them to taste good.  I also find asking him to do things with me and the kids, i.e. the zoo, will get him up and walking and active.  

I am hoping by starting off slowly, he will begin to see the benefits of it, and start to make some progress.  All I can do is hope that by not pushing too hard, and keep trying I might get somewhere with him.  

If I were you, I'd write that letter.  Be as open as you were in this post, and you might get somewhere with him.  The odds are against him changing, but by writing that letter, you'll know you gave it your all.

Good luck.

My father is an ex-marine as well.  I think it's a combination of that and his age that makes him so stubborn and resistant to change.

Thanks for the responses everyone.

I would have to disagree with the letter writing IMO. 

If he already knows everyone disapproves of his eating choices and has made the choice not to change, the only thing a letter is going to do is make you feel better about yourself, you can rest easy knowing you tried.

I have a parent in the same boat only 10 years younger, she knows what to do, but if history is a good predictor nothing will ever change. The only way she will get healthy is if she decides to on her own, outside involvement is not going to change anything.

Also it is possible that a letter or "nagging" might cause a rift on the subject so if he ever does get his own motivation it will be harder for you to be involved at that point.

Just my opinion, but I would leave it alone.