Vicks could be part of the problem. It seems to help some kids, but studies have shown it actually increases congestion for quite a few kids. I can't stand to be around anything mentholated myself – gives me asthma and gets me coughing even if I wasn't before.
I've heard Samubcus syrup (elderberry syrup) recommended – available in health food sections. I've never tried it. I believe it is said to build the child's immune response.
Honey can help soothe throats, and reportedly might help quiet coughs.
If some of the cough is from post-nasal drip, the saline should help. There's a fine-mist nasal spray for young children that they may tolerate better.
You don't say what the cause is for his cough. If it was a cold or flu, it might be wise to have a pediatrician check it. Kids can get secondary infections and pneumonia.