1) Since no one can do something (getting healthier) like this "for" someone else, he should set up an account of his own - so the two of you can work together on Spark. You can then interact with each other and with other members of SP toward the same goals - health!
2) For a moment, put his idea - wanting you to gain weight (!) - away. Focus on you: what do YOU want? To lose weight? To stay the same? To gain some weight? If your "want" doesn't match his, perhaps you need to decide how far you're willing to go to give in to someone else's idea of how you should take care of your body.