"... Remember that, because if you insist I eat more than this, I'm going to be sick and feel uncomfortable, and no one wants that."
I really disagree with this.
People can insist all they want. They have no power. The guy with the *fork* and the *mouth* has the power.
He will not be sick and uncomfortable no matter how much they insist. Because he will not eat it.
Surely he has the backbone to ignore their insisting. (OK, I get that it's hard, but I've dieted for weight loss, and I'm on a health diet now--and people can insist, but I simply weasel out--"be Teflon," I think.)
As far as how to handle it--I might start from an informational point of view, when it's not dinner time. And explain how this changes how much he can eat and *what* he can eat. I love the banana thing--if there's some physical thing he can use to demonstrate, that's even better.
Then, maybe he brings his own plate or bowl to use, so everyone can see that he's taken all the food his stomach can handle.
I did once deal w/ my MIL insisting about whether I take more food by zeroing in on the *emotional* message behind it.
I said, "Please believe that you have made me feel very comfortable in your home. When I am here, I feel loved and welcome. I feel 'treated.' I feel well taken care of. You have achieved this purpose. In fact, I feel so welcome that if I wanted more food, I would feel very comfortable to take some more.
"I think what's what you're trying to achieve by insisting I take more food. You want to be generous.
"But the interesting thing is that you are getting the opposite effect. I start to feel pressured. And then I don't -want- to be at your table.
"I'm a grownup, and I know how much I can or can't eat. And I do feel loved and welcome here. So please--don't comment on how much I eat, and don't pressure me to eat more food."
Then every time afterward, I started to say, "I already feel welcome--please don't pressure me."
After a little while she started to interrupt her own self, and then eventually she stopped completely.
All the while, I made sure she knew how successfully she was achieving her underlying emotional goal: to make me feel loved.
At no time did I criticize her. I just reassured her, and then I explained how it was being counterproductive.