For many men, the chest signifies masculinity and targeting it is a huge turn on. It is a great erogenous zone - massage it, kiss it and caress it.
The nipples are very sensitive and should be sucked and nibbled on.
Start out gently and then try a firmer approach. Watch for his reaction - some men love it rough but not all, so feel your way gently.
Learn about the erogenous zones on a woman here.