I would pack something from home and also (to round out his meal and to honor the fact it's someone's business) order him something tiny that he will eat (he really won't even eat plain white rice?).
We used to do this at our favorite Japanese restaurant when my daughter was little -- bring her some food from home but order her a bowl of white steamed rice. That way she also got something brought to her when we did -- which is fun for a kid. You can eat it yourself if your son doesn't.
I would not do McDonald's or other fast food because the food smell is pretty strong from fast foods (fried stuff or even grilled emits quite a smell that will clearly be "outside food"), and that plus hauling in the crinkling McD's bag is, well, a sure attention-getter and heads will all turn toward you, including the owners' heads. Bring some small toys he hasn't seen before, instead. This is a good time to use "PBB" as my friend calls it -- Parenting By Bribery! Something small (and not noisy) but new would keep him more occupied. Also bring old favorites that you use when out with him.
I hope your meal and especially your talk go well! You are very thoughtful and sensitive to think about this issue and to want to be considerate toward the restaurant owners.