Get boss on board with unit tests

I am reading a great book called Working Effectively With Legacy Code and really want to make some changes for some our legacy systems to make them more testable, but have a problem. My immediate manager doesn’t see the value in unit tests and has said that if they increase the development time we should not write them. What can I do to get him on board? Other managers at the company have their devs write unit tests, we are one of the few teams that don’t.