My 19month boy won't eat proper meals ie breakfast, lunch and dinner... what can I do?

It's frustrating but very normal. The simplest thing to do is make a meal that the whole family enjoys, serve him his portion at the table and if he eats great, if not then set it aside until he is hungry. No fuss, no bribing. When he is hungry he will eat and should get what was offered the first time, not what he wants. Limit snacks, possibly avoiding them if they come too close to meal time or cutting the amount to smaller portions. Hydration is very important but sometimes we can fill up on liquid making us not hungry. During the summer months this is very normal as the body needs extra hydration. He may also know that he will get that last feeding of formula to satisfy his hunger so doesn't want to eat preferring the bottle. I suggest slowly cutting back the amount in the bottle until he doesn't want it. The multi vitamin is solely your decision. As long as he is eating goods foods (fruit veggies whole grains lean protein low fat dairy and good fats) when he is eating then I wouldn't worry. If you know his diet isn't as healthy as it should be then maybe consider it.