I'm working for small company, which operates in the automation industry.
The boss hired me because he wants to sell/give some desktop applications to his
current costumers.
He imposes me to use the Netbeans Platform (a generic desktop application framework).
A software engineer friend of his advised him to choose this framework. 
At the moment I created 3 desktop applications with Netbeans Platform.
I like Netbeans Platfom. I really take advantage of modularity, Window System and Lookup.
Unfortunately I'm frustrated to known that I can do the same works with Python and PyQt in a fraction of time.
I've already illustrated to my boss the main advantages of Python, but he doesn't like the
idea to use a language that he never heared of it.
I'm the only programmer who codes desktop applications. And except the framework imposition, I'm free to use whatever I want.
I'm looking for good motivations to convince him to leave Netbeans Platform for Python/PyQt.
P.S: My english is bad, sorry.
I would guess a lot, in terms of risk management, would depend on the separation/isolation of the various softwares you develop, and their life cycle.
If you don't need to further a central set of libraries, or have (or can develop) Python bindings for those, and the projects are relatively self contained, say a turn around of two to six months, you could give him a quote for a project in Java that is reasonable and he's familiar with (to make sure it doesn't appear artificially inflated). Then give a much reduced quote for the same in py+pyQt, and see if you can get him to invest on your advice.
Without tangible evidence coming from inside that a change in route will bring benefit the more management and economics savvy people who are technically ignorant will not buy into a new platform when the old one never prevented from realizing and selling.
Without a decent assessment of why he doesn't want to switch platform and what he considers risks it's kinda hard to give more pertinent advice.
How to convince boss to substitute Java/Netbeans Platform for Python/PyQt?