A.P.
How old is your son again?
I can assure you with almost total confidence that since you are breastfeeding, he is getting everything that he needs. Breastmilk is truly nature's perfect, most whole food. It knows exactly what he needs.
If you're supplementing w/ a WHOLE FOOD (ie: not synthetic!) vitamin, that's totally fine too & will help to fill in any missing "gaps", etc... that may arrise.
How has his weight gain been? How about his overall developmental goals?--Is he staying on his own track more/less?
Hang in there, mama! He'll start eating more when he feels that it's safe for him to do so!