What should I do when my project manager does not care about implementation details?
The project manager (PM) should not care about implementation details. That's not their job, but rather the job of the software developers. Ultimately, the responsibilities of a project manager are to control the cost, schedule, scope, and quality of a project. Depending on the process methodology used, the PM might also be the primary point of contact with the customer. On a regular basis, the PM is responsible for coordinating requirements, design, implementation, testing, and maintenance activities. The job might also entail business perspectives - obtaining funding, contractual obligations, and so on.
The PM considers the project as a whole, not as the individual components. Those are up to the people in the functional roles as well as the people leading those function roles. In short, there's nothing wrong (and, I might even argue that there's everything right) with a project manager who isn't concerned with implementation details.
Will telling him that this is not easy make him see me as being lazy and a slacker?
Absolutely not. Although your PM has a software development background, he probably doesn't do that much software development anymore, if any. It's the responsibility of the engineer to estimate task length and difficulty.
In some cases, you might be given a deadline to complete the task in. If this deadline is inappropriate for the task, it's the responsibility of the engineer to inform the project manager and/or appropriate supervisors of the problem, associated risks, and reach a resolution. It might not be the best resolution, but it's important for everyone to be on the same page when it comes to decisions.
Do you feel that given requirements in the example can be expected from a single developer?
Just about any requirement can be completed by a single developer, given that they have enough experience and knowledge. However, the length of time needed and the quality of the output will change. A developer with experience in the particular domain and technologies can design, implement, and test a solution much quicker than a developer with limited experience. It's not possible to say if you can finish this assignment in the given length of time or not.