The fact that your child shrugs his shoulder when asked what they did in school does not necessarily mean he does not know what is going on inside his classroom. As Tweithof mentioned we do not know his age, and that might make a difference in terms of what he understands. However, kids at all ages might shrug of a question about school from the parent. As Tweithof says, they've been at it all day. Also, some little ones don't have the articulation skills to really summarize it for you, and it is too much effort for them. I like the suggestions in Tweithof's answer. You might want to try them.
You might also want to contact the teacher and get an idea of how your child actually behaves in the class. Is he engaged? Does he participate? Does he interact with the other kids? This might help you know if he really doesn't have any idea of what is going on, or maybe he, like many kids, just doesn't want to say. It is also possible that in this case, "No news is good news," meaning that if your child was seriously unaware of what is going on, hopefully, the teacher would have contacted you.
Another idea would be to actually volunteer to do something in the class or in the school. Then you will know first hand a little about what is going on in the school/class. Then you can ask more specific questions that might be easier to answer, like what they are working on, what the teacher was like today, etc. Then when he comes home you have a jumping off point for a discussion that might more easily bring him in.
I would encourage you not to worry too much. Talk to the teacher.
Go in, and see for yourself. And try some of the other ideas suggested.
Not knowing how old he is makes this hard to answer, but I would make a game out of it or an expectation. Tell him each day he has to tell you three things that happened at school that day. If you want you can then you'll tell him three things that you did while he was away. Don't let him move on without telling you the three things and if he chooses not to, then ask him to think about it for a few minutes and you'll ask again. The good thing is, when he does give you the three things, sometimes they can be conversation starters that will lead to more things. Keep in mind, things are pretty routine at school you may want to hear about it, but he does it everyday so unless there is something exciting that happened, it's same old, same old to him.