Adopt the phrase "You get what you get and you don't throw a fit", it's pretty straight forward. You supplied the underwear/food/plate/whatever, that's what there is, if he doesn't like then he can go without, and you don't want to hear complaining because you OFFERED.
PICK you battles, underwear would not be a battle in our house because I wouldn't let it...my son went through a commando phase-it only took ONE time of zipping up his privates to decide that mom WAS right and underwear were necessary.
If he doesn't eat what's offered, too bad! He won't starve and WILL realize that he'd rather eat, period, than eat off of a paper plate with a plastic spoon, etc. (I've got nothing on the McDonalds thing, I don't care for it, either, but I DID go hungry as a child when MY family went there and I'm fine)
Don't force him, let him make his own decisions and learn to live with the natural consequences of those decisions.
There are plenty of things that parents need to be firm on (safety, manners, school attendance, etc.) but there are a lot of things that life has a way of teaching on its own.