Mleigh - First, I am VERY impressed your h is even aware of who is president; that fog can be vicious!
In a non MLC world, I would joke, as it is my nature. I would probably tell h he was free to write a competing book report. Ha ha. However, I have tried humor with my h and it caused him to monster terribly! I have found it is very true, MLCers have no sense of humor. Just MHO.
So personally, I would stay silent just a bit and see if your h calms down about it? Maybe he will forget all about it; my h would!! Or maybe your s will change his mind all on his own? Things can change still. But, the reality is, your h may very well sway your son all on his own!
In a lot of these scenarios, I encourage my kids to do it their way and tell them it is okay to disagree with people. Eventually it will have to be something they work out on their own. Your son may learn this and kind of avoid telling your h those topics on which they do not agree. Both my kids have learned this as h is often the very opinionated 15 year old. These days I would not suggest much to my h as he can be a punk teenager but your h seems kinder.
Me 41, H 47, M 15 yrs, S11, S13 BD 1: 11/4/14 we work on it; really I pretzel myself BD 2: 3/31/15 H goes down to "dorm room" 8/15: H back to MBR 10/15: H back in dorm room 1/18: H files, now divorced