Help for a personal-trainer-by-default?

So after a long school year of many sedentary hours filled with reading, writing, and fatty takeout, my boyfriend decided at the beginning of this summer that he was hugely overweight and needed to lose the weight he'd gained over the year NOW. The fact is that he did look sadly out of shape and had put on some weight, but was not in nearly as awful condition as he thought he was -- but that's not really the point here. The matter at hand is that he turned to me to be his personal trainer. I was intensely involved in sports all through middle school and high school and was a D1 rower before transferring; I know a thing or two about training at this point. That said, I am a 5'8" female. It has been a very long time since I have done workouts not designed for girls or for people who are already in really good shape. I have NO IDEA how to put together a workout appropriate for a 6'2" male who has never lifted before in his life (golfers...) and whose youth is the only thing keeping his cardiovascular fitness at a decent level. My original plan of emphasizing bodyweight exercises and building an aerobic base seems to have gone awry.

Any input on how to help him build up his fitness safely and sustainably would be tremendously appreciated.