My Manager is blocking my IJP

I'm a bizdev/client servicing person wanting to transition to product management. It has been almost 2 years in my current company. Overall I have ~7 yrs exp

I applied for an IJP for a product role, got shortlisted for interview with Product head, who is also the hiring manager.

The HR scheduled an interview but cancelled it last moment, citing manager approval as the reason.

They sent an email to my manager, without my prior knowledge, about my interest in IJP and asked for his approval. He didn't approve.

I spoke to him, trying to understand the reason and got scolded that a discussion with him should have been the first step - about my long term career goals and aspirations and what not. And told me that I should spend at least 24 months in a role to justify my learnings and career. That I should think about that first in my current role and forget about this product mgmt transition thing.

So first of all, there is no such rule of spending 24 months in a role to be eligible for an IJP. HR just needs my manager's approval to proceed.

And secondly, the work in my current role is boring and not exciting at all. I honestly feel like I've wasted 2 yrs of my career by joining this company, and the only reason I did it was bcoz I got wfh.

One quick context. My manager is a senior level person who heads 2 business units. One is performing poorly, and the other is doing comparatively ok, in which there are only 2 team members - him & I. So I feel that him blocking my IJP is so that his position in the company is not jeopardised or at least not affected negatively in any way.

This is not a rant. I want your help to understand how I can get out of this situation, and get a chance to proceed with the product IJP