Fetch data from data base for that specific user and set Meta like this......
string[] keywords = { "KeyWord_1", "KeyWord_2", "KeyWord_3" };
HtmlMeta htmlMeta;
foreach (string keyword in keywords)
{
htmlMeta = new HtmlMeta();
htmlMeta.HttpEquiv = "keywords";
htmlMeta.Content = keyword;
this.Page.Header.Controls.Add(htmlMeta);
}