Is excluding a co-author from a proposal considered academic misconduct or just an ethical issue?
Your question as I understand it is: is it considered misconduct to exclude a co-author from a proposal, or is it some other ethical issue?

Certainly "research misconduct" can broadly be defined as "ethically problematic behaviors in research" - in which case there is no difference. However, let us assume a narrower definition of "misconduct."

The specific definition of "misconduct" may depend on where you are. In the United States, the Office of Research Integrity (ORI) defines "research misconduct" as follows:


  Research misconduct means fabrication, falsification, or plagiarism in proposing, performing, or reviewing research, or in reporting research results.


and "plagiarism" is further defined as 


  the appropriation of another person's ideas, processes, results, or words without giving appropriate credit.


(Individual institutions may, and often do, extend this definition as a matter of internal policy. However, the ORI policy represents the least inclusive definition of misconduct in the United States.)

According to this definition, it seems that using your coworker's ideas, processes, results, or words in your research proposal without giving him credit is academic misconduct.

Having said that, the ORI also says this on plagiarism:


  Many allegations of plagiarism involve disputes among former collaborators who participated jointly in the development or conduct of a research project, but who subsequently went their separate ways and made independent use of the jointly developed concepts, methods, descriptive language, or other product of the joint effort. The ownership of the intellectual property in many such situations is seldom clear, and the collaborative history among the scientists often supports a presumption of implied consent to use the products of the collaboration by any of the former collaborators.
  
  For this reason, ORI considers many such disputes to be authorship or credit disputes rather than plagiarism. Such disputes are referred to PHS agencies and extramural institutions for resolution.


From your description, however, it seems that this is not the case in your scenario - you and your coworker did not "go separate ways." There is no "implied consent" to use his ideas. He fully expected to be a co-author, and you yourself admit that he should have been a co-author, except that you somehow left him off. 

(Even if this is considered an "authorship dispute" and not "plagiarism," it's not up to you to make that determination - it's up to the investigative body, who will look into the matter if your coworker chooses to report this.)

Finally, "honest error" is not considered misconduct according to the ORI. However, your coworker is not required to take your word for it that your exclusion of him from the proposal was an honest error. This is a determination that a third-party investigative body will typically make.

In summary: Your coworker is well within his rights to report you for misconduct. If he does so, you are free to try and persuade the investigators that this was an honest mistake.