My aunt went through similar things with her daughter. The kid was about 3 or 4 and my aunt needed a couple of people to hold the kid while she gave her the medicine with a mouth syringe. The kid screamed blue murder, but some kids just will not take medicine, and while forcing it down their throats doesn't make them want it more, it's sometimes the only way to get it into their bodies.
How do you give your child the cough syrup? If you aren't already using one, I suggest using a mouth syringe. Or perhaps you could use a scare tactic and tell him that if he doesn't take it through his mouth that he'll get it in his arm through a needle. Or maybe try positive reinforcement, like telling him he's a big strong boy when he takes it or whatever.
From what I have observed, holding of the nose doesn't help make medicine taste any less rancid. I've tried it myself when I've had to down some pretty disgusting antibiotics and it never worked.