It is extremely unlikely that he doesn't understand the dangers of obesity to his health, considering the wealth of information available even to some of the lest informed people around. It is unlikely (although not as much) that he does not understand the reasons for him being fat, i.e. the surplus of calories. In case he in fact does not understand this, you should teach him, although I strongly suspect that you have tried this already. If you haven't, try teaching him about it in a way that does not relate to his situation specifically (instead of "you are fat because of...", use "people gain and lose weight because of..."), so that he does not interpret the information as part of "another one of his scoldings" and block it out.
So why is he fat? He is fat because his calorie input is greater than his calorie expenditure, that part is obvious. But why is that? He eats too many calories, and exercises too little. You need him to eat less calories and exercise more. I am aware of the obviousness of what I'm saying, I'm just trying to draw a clear and linear train of thought.
Why does he eat so many calories? His over-eating might be rooted in emotional issues, so that he seeks comfort in food. In that case, rudely motivating him and making him feel bad about himself in unlikely to help because it will increase his anxiety and self-hatred. It might also be that he looks for enjoyment in tasty foods and it happens that the tastier foods are also the most caloric ones. In that case, he is used to consuming foods with so much sugar, sodium and flavoring that he is almost unable to taste "regular" food; you can help by changing his food habits by introducing him to foods that are both tasty and healthy, slowly de-conditioning him from his current habits and helping him regain his sense of taste. Learning to cook together can be fun and good bonding experience.
How can you help him exercise more? He needs three things in order to exercise properly: knowledge, discipline and motivation.
He needs knowledge in order to exercise correctly in a way that will both have results and not cause injuries. Without knowing how to exercise, he can put in a great amount of effort with little to to results, and/or he can get seriously hurt; two things that will undoubtedly demotivate him.
Discipline tends to be the trickiest part, and people often think that one can rely on motivation alone, which isn't true. Discipline is achieved by instilling him the idea of committing to his routine and not giving up when things get hard. This is a lot easier if he already performs some activity that requires discipline, be it painting, programming or playing an instrument. Use this to explain to him that just like that activity, he will start by being really bad at it and not believing that he can get better, but over time and with discipline he can achieve amazing results, results he himself did not believe that he could achieve. Paint a clear picture by drawing a parallel between something he used to do poorly that he now can do well, thanks to his perseverance. If he does not engage in any activity that requires discipline, I'm not sure how to proceed. Perhaps take on my previous suggestion of learning how to cook together?
Finally, there is motivation. This is a relatively simple matter: find something he wants that requires him to lose weight (women, being able to play sports), and remind him of it. Alternatively, find something he doesn't want that will happen if he doesn't lose weight (diabeetus, death). He is likely already aware of the risks for him, but has he thought of the people who will suffer with him? Maybe his weight makes him think less of himself, and so he doesn't think dying would be so bad. Don't appeal to his sense of self-preservation, but to his empathy: how would your parents feel if he died before them? How would his friends feel? You obviously care about him and will be broken up if he dies an early avoidable death, does he not care about that? How you present these ideas is up to you, of course.
I hope I could be of some help, best of luck to you both.