At 3.5, he could live just fine without store-bought drinks. After all, Rice Dream, or Almond milk, Coconut Milk, etc. are not a necessary part of a person's diet, and often people buy them sweetened (not healthy). But, that said, you can get powdered non-dairy milk-replacement formulas if you are concerned and want him to have his "milk." Ask his pediatrician about it. These others are usually not "milk equivalents" or "milk replacements."
If it is just about calcium you are concerned about, there are better sources such as fish (sardines, salmon, perch, trout, and tuna), dark green leafy vegetables, blackstrap molasses, some types of beans... then there are fortified foods, which you may not be able to get in India. You might ask the pediatrician about him taking a chewable multi-vitamin.
For the long flight, I would bring lots of fruits, veggies he can slowly eat, a couple sandwiches he likes cut up into smaller sandwiches - make them out of things like almond butter and jam, melt Daiya "cheese" on it (to make it stick to the bread - it is fine eaten later), Better than Cream Cheese (non-dairy but has soy).
For fluid, water works great. Or for a liquid snack, bring a couple of the little boxes of the soy, almond, coconut, or rice based "milk" drinks (most of these are not nutritionally equivalent to dairy milk - we just call them "milk") or - the powdered non-dairy formula stuff to mix in water (which is closer to what nature intended human babies to drink).