Tell your boss you're taking this to the CEO, but welcome the opportunity for him to stop or guide you. There are a few reasons this idea may not have moved forward despite the initial positive feedback you got:
- It's not actually worth doing. The people you've talked to may have felt good about the idea when talking to you. Perhaps it has some real potential, but it doesn't help the company as much as other things they're working on. Perhaps it's something they tried before and they don't think it'll work. Perhaps it's wrong in a way that's obvious to them but not to you. In any of these cases, they were probably still impressed by your initiative and creativity, and perhaps they were even deliberate about choosing a delayed response instead of an immediate one, because telling someone that an idea they care about won't work can be super demoralizing. If you can get some additional information from them before taking this to the CEO, you can potentially improve the way you present it, or maybe spare yourself the embarrassment of presenting something that won't look good.
- They don't want to own it. Maybe it's a great idea for the company, but not something they feel ready to execute. It's often hard, in an established company, to make time for risky new ideas even though those can be critically important. There are also more banal issues, like the challenge of how to compensate you fairly for an idea that you had before you started but likely needs substantial refinement to launch. Often CEOs want more risk-taking and innovation, but middle managers have good reasons to be cautious because failing at an attempted new project can reflect poorly on them, and the nature of new projects is that there's a big risk of failure. In this case, your manager might be happy for you to take it to the CEO, but they also might worry about how it reflects on them - perhaps there's a way for them to make a warm intro so that this idea can reflect positively on both you and your manager.
- It's good but conflicts with their other goals. This is the worst-case scenario, and covers situations like your manager feeling that he cannot let you take credit for the idea, or worrying that his own failure to suggest the idea would signal incompetence. In these scenarios, which are probably rarer than you think, it may be a good idea for you to actively circumvent your manager, but it will definitely be risky.
- It's just not urgent. They're busy and hope to get to this someday, but it's not an "on fire" problem like the 20 other things vying for their attention.
In most of these cases, it's beneficial to you to get more information from your immediate manager before you go to the CEO. But for that to work, you need to be open to feedback, including the possibility that your idea is seriously flawed. This can be a great opportunity for you to learn -- about the idea, the company, and how to best deal with your managers and colleagues.