why go with all or nothing? Shouldn't he be able to use it piecemeal perhaps on any new code he is developing? Dependency injection is great, but the old school factory patterns compensated for it pretty good IMHO, so +Max Völkel
you should at least start coding to interfaces going forward. I was happy how much boiler plate code GIN/GUICE eliminated for me, kept code more readable as well, and much less interdependence between objects.