The best milk you can add to your baby's food is breast milk. you can mixed the rice with breast milk (if you are breast feeding ) . Next best thing you can add is a spoon of oil. Add vegetable/coconut oil, butter or margarine and cook it. It added tasty to the food and most importantly these oils/butter contains lipids
which have higher calories density compare to protein and carbohydrate
It's very common complains of mothers that baby are not taking food. so don't panic. complementary feeding is continuous process. you can start with plain rice and add new item to rice in every 5 to 6 days time. Remember to introduce only one new food at a time to the baby. If he tolerate that food you can add another one in 5-6 days time. Baby should experience different taste and different colors of foods. That how you should improve his appetite.