Tell them about the head start you'll get from working with an established code base. Show them how many proven tools are available. Explain that they'll be able to rely on the worldwide open-source community when they need to answer a technical question or find a replacement for a departing team member. Team leaders know they are judged on whether they can meet deadlines without a lot of expensive overtime. They want the project to run smoothly.
Open source is a good match for what team leaders want.
Developers get caught up in the intellectual excitement of new technology, but an IT manager never does. More than anyone else in the organization, the IT director understands the value of proven tools, proven architectures and proven approaches. IT directors understand the cost of change, including indirect costs, and they understand that resources are finite. They function as gatekeepers, withholding resources and approval from all but essential initiatives that earn their grudging support. To convince the IT director, outline the dangers of sticking with the proprietary-software approach.