Have his favorite drink or food ready - something to make the taste go away FAST, and it's also a reward for taking the medicine.
Be honest - it tastes yucky, even grownups have to take yucky medicine, but it's to make the germs that make you sick get out of your body.
And give him the power. This worked like a charm for us, and I started doing it at this age.
If you have th syringe thing - most pharmacies give them for kids - put the medicine in it and give it to HIM so he can give it to HIMSELF. He can practice with water or something. He can give water to you with it. Let HIM do it.
He HAS to take it, but if HE can give it to himself, it's not a battle. You're supporting him. And seriously, have that drink or yogurt or whatever RIGHT THERE so as soon as that medicine is in his mouth, he can shove the good tasting stuff in there to make the taste go away.
And praise the heck out of him for "doing such a good job taking care of himself, even though it tasted yucky" - because that's what he did :)