This has happened to me. Since he isn't totally observant (and there is an endless variety of what people actually do or don't do when they call themselves Kosher), I would let it pass. Really, I know people who are "Kosher inside the home," but go out for Chinese food and eat shrimp. It sounds like he would understand if you told him, but I would judge what to do by what you think his reaction will be.
It's thoughtful of you to think of this, though.