I would recommend that you get the weight and height checked for the baby first. Of the growth is normal and the pediatrician is happy, then possibly you are over concerned, like many mother's in or practice!
If the child remains a picky eater, I would suggest gummy vitamins for the same. These are like candies and easy to give to kids.
A good brands is Chubears. It is available from Amazon India website or can be bought at WWW.Chubears.com as well.
Commercial powders like bournvita etc contain too much sugar and are thus less useful.
The probable reason for his decreased intake is probably milk prediminant diet. You as parents and the child after 1 year of age have to grow out of the milk predominant diet plan.
Just give milk only once in day time, may be at supper time and not the first thing in the morning. Rest of the day make him eat whatever you cook in the house.
Meet a pediatrician for some iron and zinc supplements to help him further.