What do you think of Zinc for colds?

I give my son zinc and vitamin C tablets daily as he suffered badly from cold sores. Since taking them he does not get them anymore and he has never had a cold either. As for Echinacea, I heard that it is good for the immune system, but it should be taken 2 weeks on, 2 weeks off to have a full affect. I am not sure how true this is, but my mum told me and she reads up on everyhing to do with natural alternative treatments. So I do tend to believe what she says.