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.
If your selling skills are not working in discussion format I highly suggest that you document it. Some managers/bosses really respond well to this.
Make a matrix of all the metrics that you yourself use to grade the two frameworks (I leave the level of objectivity to you there: for example if objective it should analyze the cost of transition and the loss of institutional experience; but it might not be high).
Finally, send it by e-mail and viola you have:

made a report/analysis of the situation providing options for improvement
this shows that you are thinking towards future and that you show initiative

EDIT:
You can also ask your boss to show your analysis to his friend if he trust his friend that much, but ask for a written counter-analysis so that you can address the critique.
It is a good thing to do it openly and document the decision process well, since ultimately, if your suggestion is accepted, you will share responsibility for the decision.
How to convince boss to substitute Java/Netbeans Platform for Python/PyQt?