Discovering your partners erogenous should be fun; all part of foreplay. Take on the task as a challenge, not as a "task". This is when one of those mutual masturbation sessions comes in handy. Since you've watched him masturbate and learned what he likes, it should be easier to find other pleasure points. A fun time to start your discovery is bath time. If you have a large tub, better yet a Jacuzzi, you can both take the time to explore while cleaning each other.
If you're not sure, most people have common pleasure points, but you are going to have to take time and find those other secret spots not mentioned in here. Working your way down and around the body, the lips are obvious erogenous zones that respond to kissing and licking. There are entire books written about kissing and using the lips, take advantage of some of them. The lips can also be used for stimulating other erogenous zones. The shoulders, neck and back muscles can be relaxed with some stroking or massaging, light kisses and licking.
A man's nipples are also very sensitive and respond to being kissed, sucked, pulled or gently pinched. You know it's working when his nipples become erect just like yours do when you're aroused. The buttocks and anus are heavily supplied with sensual nerve endings, and many men will become aroused with caressing in this area. The anus responds to any sort of touch, but some men find it uncomfortable being touched in that area. This one very important reason why you should watch you partner's reaction to caresses you're making.
The fingers and toes are quite sexy parts of the body and many men find it exciting to have a woman suck, lick and kiss their fingers, feet and toes. The fingers are more of a visual turn-on for a guy when he sees them being sucked. It's a move that really gets them at the groin in that it resembles oral sex. Flicking someone off with your middle finger is a way of saying "F***You" and sucking the middle finger is like sucking that particular part of the body which it resembles.
The male genital area is a sensitive area period. Usually it's best to save that sort of e-zone discovery for last. Unless your partner has exceptional control, it's easy to set him off with caresses to his genital region. The scrotum is highly sensitive to pleasure and pain, so some men don't like it being toyed with. A trusting partner would allow you to fondle his testies. The base of the penis is also susceptible to touching; however the penis is the most sensitive. The entire length of the penis is sensitive, but the glans at the tip is packed with nerves. The frenulum and the area just behind the opening to the urethra also respond to touching, licking and kissing. If you're not sure about what some of these areas are referring to, take a look in the male reproductive area for a picture of the penis gland