Let him 'get away with' not going to school, and to 'be there for you' the day of your surgery.
IF he'll agree to 'be there' for you.
I can nearly guarantee he will be a young man that day.
And if you forced him to go to school, it would be exactly as PAlady said - a wasted day worrying and at the VERY MOST getting into trouble a few times.
But only allow it if he'll do exactly what he wants to do anyway: be a caring young man for his mom.
And be sure to reign him in on daughter in the meantime. Once you give the OK, don't be surprised if he starts trying to control her right away.
I say these things cause that was me in a similar circumstance at about
the same age. And I was the 'Man of the House' for about
a week in an effort to control me.
Wife: Liz, the choice of a lifetime
Dogs: Koshka Prayer
& Chomp Prayer