What repercussions do I risk if I exclude my postdoc advisor from list of co-authors against his will?

His participation in this work [...] was financing me, supervising and editing the draft. The research design and implementation is entirely mine. The original idea was proposed by some external collaborators.

From what I see, I think that your supervisor deserves an authorship of this paper. I think that you also should at least offer the external collaborator a chance to contribute to the paper and become a co-author, too.

• Your supervisor (PI) was working on a particular topic and presumably is an expert in this field.
• Your PI has an established network of connections, and was able to realise that the idea of external collaborator can work for the problem you are working on. The decision to apply a particular idea to a particular problem is a risky decision, and (in case of success) a significant contribution.
• Your PI hired you. This is a risky decision, which itself is a part of a wider research process. The choice of people working on the research project is itself a part of "research design".
• Your PI listened to your ideas (at least during the interview) and approved them. This is a part of "research design".
• Presumably, you had a few research meetings with your PI, in which he listened to your ideas, corrected them and approved them. This is a part of "research design" (which is a continuous process).

The fact that your original manuscript was rejected (for which you are the principal author, I assume) can indicate that you are not yet prepared to write to a standard of independent academic researcher. That's why an advice your PI gives you is important and valuable. For this supervision process, and also formal editing on the manuscript, he (or she) deserves to be included as an author.

Now, to answer your question. If at this stage you decide not to follow your PI's lead, you will not benefit from his advice and support any more. Based on what you've said, it looks like this decision can significantly reduce your chance of becoming a successful academic researcher. A particular repercussion take a form of bad recommendation letters, cut off funding, lack of support in internal promotion process, cut off access to your PI's network of collaborators. But most importantly, it is lack of his personal support and advice. Academia is a complicated world, and many early career researchers (including some young professors) can not navigate themselves through it efficiently without a help of their mentor/supervisor/PI. The lack of this help is the most serious repercussion. Do not underestimate it.