I see lots of options.
1. Say "I had gastric bypass surgery a few months ago so I'm not a big eater these days. How about going out for coffee instead?"
2. Say "I had stomach surgery a few months ago so I'm not a big eater..."
3. Say "I don't have a big appetite. Could we just get coffee?"
Substitute any other suggestion for coffee!
I wouldn't make a big deal about your surgery, but I would probably tell him. Just like, if a date asked me if I wanted to go hiking, I would say, well, I have a herniated disk in my back and can't walk to far. Could we go see a show instead?
Disclaimer: I am not a doctor and do not play one on TV. I've done a lot of research on vitamins but am not qualified to give medical advice. I'm happy to share my research with you, but you should see a health care professional if you want medical advice.
Check out my blog at: storyofmyservicedog.blogspot.com/