A long time I was thinking of ways the government might help to promote the use of open-source software (OSS). Now I realize I was looking at the issue from the wrong side. Proprietary software depends on the enforcement of intellectual property rights by the government.
Without the police power of the state and publicly funded courts, there is no intellectual property. It follows that the government could stimulate OSS simply by revoking its recognition of proprietary software. I admit this would be an extreme action and hence I would propose a more moderate policy.
Proprietary software will continue as a legal construct, but the developers will be required to pay a 15% tax on the royalties they earn. This is justified by the benefit principle of taxation, to cover government expenses on the enforcement of proprietary software.
Developers of OSS will not be required this royalty tax as they do not rely upon the government for the enforcement of intellectual rights. To be true, no software developer will have to pay this tax but if he refuses to pay, then his software will be open-source by default.