Valerie,
Once someone starts with cardio Coach they usually end up getting all if not most volumes!
If he is in pretty good shape Vol 4 has the most fabulous music and a great mix of challenges in my opinion. They all have great music - Vol 4/8 just happen to be most inspiring to me. I recommend to start with an earlier volume (pre 5) just so, if he likes it, he can experience them progressively.
Vol 2 is my most used shorter volume and it still kicks my butt. Like Michele said as you get more fit, any Volume of CC will still give you a great workout because you can increase your settings.
The ones I save for days when I have a lot of energy and Vol 8 and Vol 7. They are customizable as shorter less instense workouts too (since they have tracks that you can skip), but I can never stop half way through a CC. Once I start I am not capable of getting off the cardio machine until the workout ends.
~* Vrinda *~