I'm inclined to agree with your MIL...you certainly don't want him to backslide and if his penis isn't getting irritated by seams/fabric of his pants, then wearing undies isn't that big of a deal. I'm surprised the boxers didn't work because they are so much like regular summer shorts....
Would it help to let him pick out underwear (when it is time to try again) to see if he would value them enough to not want to potty in them???
Enjoy the little things, for one day you may look back and realize they were big things. -- Robert Brault
"We either make ourselves miserable,
or we make ourselves strong.
The amount of work is the same."
"The will of God will never take you where the Grace of God will not protect you."