I lived and worked in China for four years. Culturally, it is VERY hard to say no to food when you are a guest, especially in someone's home.
I would be upfront with your friend before you even go over. Tell him that you are working on losing weight and you really don't eat much. Tell him you are worried that you will offend him and his wife if you don't eat a lot, but you really CAN'T eat that much (use the word can't, not won't).
When you go over, stick to your guns. Maybe even remind them that you are not eating a lot . . . but, to be polite, compliment everything! Tell them how good it is and how much you wish you could eat more, but you can't. Eat very, very slowly, and do NOT clean your plate or they will put more on it. Tell them you could not eat one more bite, you ae so full.
If they offer, take some home, tell them you will eat it all over the next few days, and then chuck it.
Also, it's very polite and respectful to take a small gift when you are visiting a Chinese home. My suggestion would be fruit - clementines or oranges are a good choice - and that becomes your dessert, which would be filling and low-cal. The gift also will lessen the impact of your not eating so much.
Man, I'm jealous! I want homemade Chinese food!