Take it slow at first just as you would if you were starting out with a new exercise program for yourself. Be especially mindful of how he moves and how he breathes normally so you will see a big difference when you are working with him. At first, anyway, err on the side of caution. You still need to get to know each other.

Remember, he needs to learn to trust you specifically and you him. So while the tendency is to want to ride "hell-bent-for-leather" S - L - O - W is the opporative word here.

Good luck!