Yuck, that stuff is nasty.
No kid wants to take it.
No kid ever wants to take any medicine, though, do they? Heck, I don't like taking medicine either.
You know what would make it worse for me? If my husband called me to the kitchen and said, "open your mouth and take this right now." Or "Open your mouth" and squirted it in without letting me choose how/when to take it.
So give your child the same courtesy. Explain that he has to take it. It isn't an option not to. But, he can hold the cup (or the syringe/dropper) and here is a cup of ___ (his favorite drink or a "special" treat kind of drink) that he can have the instant he has swallowed the medicine. It will help wash away the taste. And then he can have a small snack afterwards if he would like. But, he must take the medicine first. He can hold the cup or you can do it for him while he holds the "chaser". Whichever way he wants. And then wait.
Let him choose. Then let him have a few moments to collect himself and screw up the courage to take the medicine.
I've always done that for my kids and they always managed to get the medicine down. No throwing it up after. No gagging. No refusals. Lots of complaining how nasty it was after the fact. Lots of quick reaches for the chaser. But the medicine always has gone in and stayed there.
It's amazing what a little control can do in a situation like that. Even now, at almost 12, my daughter needs a few moments to drink even a dose of cough medicine. So what? What is 30 seconds, or two minutes? She takes it.
There is a lot to be said for how you present stuff like this to your kids. There is nothing optional about it. But, they DO get the option to decide how to take it, and how long they stand there looking at it first. My son is one of those "get it over with" types. Same thing with eating 2 bites of a food he doesn't like. My daughter, on the other hand, is a "stare it to death first" type. And she leaves the required 2 bites of her food on the plate until she is done with the rest of her dinner, too. But, they always end up complying. On their own terms.