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.
The basic problem here is that your non-technical boss is getting conflicting advice from you and from the friend who advised him in the first place.  If you want him to take your advice seriously you need to prove that your advice is likely to be trustworthy.  And that will only come with taking the lead and being successful with significant projects in the company.  Right now, you haven't earned his confidence.
The other thing to consider is how your preferences mesh with the company's objectives.  For instance, you want to be able to write code fast.  But the boss / the company needs code that is going to be reliable and maintainable ... if you decide to take another position.  He doesn't want to be left in the awkward situation where the company is contractually committed to deliver code that doesn't really work properly, and the only person who understands it has left.
How to convince boss to substitute Java/Netbeans Platform for Python/PyQt?