I have two things I've used with varying success:

DS loves to tell me I'm wrong, using that drive combined with humor sometimes worked for me last year when he had clammed up.
Me: So how was school?
DS: ....
Me: That good?
DS:.....
Me: Want a juicebox?
(typically the exchange would just end at that point)

Me: Something must have happened for you to have that look. Hmm, DS, did you bite Melissa again? (this happened when he was 18 months or so, tone is one of fake seriousness.)
DS: NO!
Me: Well then did you beat up X? (fake serious tone, X is child in wheelchair)
DS: No. (looks at me like I'm an idiot)
Me: Get arrested?
DS:.....
Me: Well then you must have at least farted on the teacher?
DS: (smiles)

The beauty of it is no response is actually needed and it still felt positive. At the very least they are distracted from their day by being disgusted with your stupid humor. As opposed to the, "So how was school" conversation that ends with just that thick empty silence. And one is hopefully getting across that the bar is set so low that parent feels they gave successful performance if they did not physically hurt anyone. Do not try with a literally minded kid.

If all had gone well that day between us then later on in the evening I might try a trade. "You want me to play more of this game, right now? Okay but first you have to tell me what story the teacher read today." (very specific question that I know will be easy to answer). As the weeks went by and he was more used to actually answering at all I could ask more vague things.

It also really helps to have inside info. Mrs. Y saw him in the library at 2pm. Shoes are not muddy so PE must have been in the gym today.

This year is going way better in terms of DS telling me stuff. I think all built on the tiniest of successful exchanges. So don't give up.

The things I end up reassuring him about now are the craziest things. For example him thinking the teacher globally likes him less because she wrote "great job" on someone else's work but nothing beyond a check mark on his.

DS6 is at his most odd Thurs and Friday, and after a long weekend is at his least odd. Simply his brain expressing an overload of stress, pathology oozes out saying, "help, this is not a tenable long term situation." Have you noticed if the oddities wax and wane like that?