I know this a really hard to do with a bad experience under your belt, but I would also suggest being upbeat about whatever stuff he brings home even if you think it is the lamest thing you have ever seen. I've said my share of noncommital comments like: "Well, let's see now. Pretty interesting. I'll be interested to see how that turns out" when thinking something far harsher! Especially since you son is having a good time, there is no reason to let your anxiety taint if for him.
I'm going to write those things on my hand or a cheat sheet. Because you are right, it isn't fair to taint his experience. I'm not really good at having a 'poker face' and react dramatically sometimes...
Okay, I'll practice being positive or neutral at best in front of him. Thank you for your advice!