16 month old cannot go past 9kg but hates vitamin drops

My boy was a picky drinker (not eater) so he often got constipation. He drinks only juice (apple or prunes) and yogurt milk. If formula milk, or water, he will just look away. Pedia adviced nutriplex, but everytime we try dropping (syringe) he will cry and have tantrums. He also knows if we mix with liquid or use a spoon. Now he averts when he knows its vitamins time. Any solution that worked for you?