milk supply issues!

My baby is going on 7 weeks this week and I've seemed to produce less and less milk as days go on. He nurses so much and I thought that the more demand, the more supply. But in my case it just doesn't seem to work that way. I want to exclusively breastfeed but I also want him to be full. Any suggestions on how to increase milk supply? Has anyone tried milkful lactation bars? Do they work?