How do I get my DH to start working out more?

I lob my husband to death. Would do anything for him. It used to not bother me that he did work out alot but now it does. I don't think I'm superficial by any means because I chose him based on his heart and not his muscle tone. I've tried the whole it's me ending to work out thing just to get him to the gym. It's not that he is overweight either. He's actually really thin. I finally for him I get a gym membership but he still really isn't taking it seriously. I'm actually staRting to get frusturated because he has lost a lot of his muscle tone  I don't want him to be super big I just want him to be strong and healthy, ya know? Can you ladies help me out on this one?