1)
Any sexual activity is secret. No public displays of affection. No air-kisses. No riding the moped in public.
2)
Don't discuss your weight with him. Let's face it, you both know it's there, so don't start bemoaning your body and pressure him into lying, i.e., "What are you talking about? You don't look fat at all."
3)
Go further than skinny girls. If you can't sell him on your body, you'd better overcompensate with sexual perks.
4)
Never, ever, ever, ever, ever push the relationship thing. Everybody knows guys hate discussing relationships, so make it easy on him. Bottom line: Let him get the milk without having to buy the cow.