What makes him your best friend? What do you like, appreciate or consider positive about him?
Other than the fact he has a joinery business (and that only in the negative context of his driving 150 yards to get there) I know nothing about him whatever except that he is morbidly obese and has a very difficult family situation where frankly it appears that he is probably making them happier (subconsciously) by continuing to kill himself. You say you nag and it seems to be unsuccessful plus makes you feel bad. But you want him to know you care so you can't stop?
So don't stop, change your method of telling him you care. Stop focusing on his weight and problems and appreciate some thing(s) consistently and openly instead. Nag him with your appreciation not your fear. Fill your relationship with what is good about it, things he does do, can do, wants to do. Feed (yes verb chosen consciously) his self esteem, feed his enjoyment of day to day life. Overeating for comfort or boredom is common, let him find other things to feed himself than food and it may not be too late.
Yes he has to decide for himself that life is worth living and want that enough to make changes. But for that he needs something to live for, times to be happy enough to want more. If his entire existence is full of criticism and neglect from others and from himself, then your best friendship gift is to create a little island of hope, not of being slim, not of losing fat, but of having a good enough to normal life, normal relationships/friendships, being accepted AS HE IS so that he can allow who he is to live and evolve through change. Changing to please others never ever works, because they don't love you more, they either love you, really you, or they don't.
So, do you love him? Right now? Or is he just an embarrassment, a walking example of how your friendship isn't enough to change him, how your success doesn't lead to his, how he drains NHS resource and what people might think of you being friends with someone like him. I would guess (without knowing him) that is how he sees himself, is it how you see him? If not, find a way of showing him what you value, what you get from him being in your life, how he fills a place in the world that would be missed. His family don't seem to be the people to do that, so maybe you, his best friend, maybe other friends if he has them, maybe you can.
The most successful life change stories come from people who find a reason to live. Find and feed, nurture, grow that and food won't be his only go-to to fill the void. Ignore the method, help him find a dream and believe in his ability to work for it. THEN ask how you can support his success, let him lead and walk beside him all the way. You can't push or drag someone's life, only be with them.