What you need to do is start using it, and ensure that everyone *else* who contributes uses it, too. That's a policy detail and needs someone to enforce it.
Bingo. Source control is an unalloyed good thing, but only if everyone is using it. All it takes is one influential holdout to bollix it up. Suppose that in an effort to get everything into a repository you do lots of work to ensure that the current production code is checked in. But a very senior, highly trusted developer has no interest in using it, so continues developing code from his personal workstation, never committing changes. Now he is on vacation, a bug pops up, someone else innocently goes to the repository, fixes and deploys the code, and loses a bunch of changes that the aforementioned highly respected engineer has already deployed. Customers are screaming, and the engineer blames the source control system (totally unfairly). Now the seed of doubt is planted in managements mind, and everybody is yelling at everybody else.
Source Control is an easy thing to sell to developers once you use it at all. But it only works if everybody is on board.