In my experience (and also based on things I've read) there's not much you should do. First of all, around this time, appetites naturally decline because they need fewer calories -- they stop growing as rapidly as in the first year. Secondly, at this age, they start seeking autonomy, and refusing to eat certain things is a way of doing this.
As another responder said, the important thing is that you are OFFERING only healthy things. Do not fall into a trap of offering sweets etc just so he will eat. Continue the breastfeeding if you want, but do not try to increase it. Has your son started milk (bottle/cup)? Offer this at various times during the day. At the 18 month appt, your pediatrician will tell you if there is reason for alarm, when he weighs your child.
In the meantime, do make sure to (try) to get your son to take whatever vitamins have been prescribed. As long as he gets the vitamins and a little bit of protein and calcium, he should be fine. According to Dr. Brazelton (whose philosophies I really like), a child this age should eat a pint of milk (or the equivalent in other dairy products), 2 oz of iron-containing protein (meat or egg,or iron-fortified cereal), one oz. fresh fruit or juice, and a mulivitamin. If a child gets this amount of nutrition daily, he is receiving a sufficient diet.