On the plus side, if a project's gone really well and is very important for the personal prestige of your line manager and the line manager is a little bit nervous about being unable to maintain and expand the thing he's getting credit for .... that sounds like a good time to ask for a pay rise or other improvements to your contract. That might be far more useful than the boss publicly conceding how little of the work was his own, and he can even spin the outcome as how effectively he saved the company money by hiring such a good quality remote subcontractor at initially below market rates!
Nobody expects honest answers when it comes to this sort of questions. If you reply honestly, you are condemning yourself to a life of 9-5 jobs. The correct answer to this sort of questions is a generic response "I am looking to explore new opportunities", or something similar.
DON'T DO IT. You are cornering your boss, implicitly showing your conflict/concern. It'd backfire on you