I'm going to tell my obese friend to lose weight and change his lifestyle
Why are you going to tell him that?
I can see him suffering because of his weight and I want to help.
It is nice that you care about your friend and want him to be healthier.
We're going to start the gym next week.
Your friend is lucky to have a person like you on their side. It is not easy to bring up the health of a person, but you did it in a non confronting way.