ORIGINAL: cfircav8r
As a plane fly's away from you it appears to descend due to the change in our viewing angle. Trim it out for straight and level and have him fly long passes without using elevator. This will teach him what level looks like as the plane approaches and fly's away. Also teaching him how to trim for cruise may help make him more comfortable with letting the trainer airplane do what it does best, fly straight and level.
Gene,
Follow this advice with one added piece. After he has made his turn and the plane is flying straight and level have him lift his thumb off of the elevator stick. If you need to enlist a another person to verify he's not touching the elevator stick. Now let him fly across the flying pattern until it's time to turn again, then he can put his thumb back on the stick. By doing this it won't take too long for him to both learn what level fligth looks like, but also learn that he doesn't have to have constant input on the stick to keep the plane flying. My bet is that within one or two flights you'll work out his tendancy to climb the plane like he's doing now.
Ken