Best food/drink to improve the quality of breast milk?

My baby is 15 weeks (4 wks prem) and keeps wanting a feed evey 1/2 hours esp. at night. I've noticed my milk is quite watery hence I think he it's not keeping him full for long. Also because he has Asthma-like symptoms he doesn't stay on for long. I must admit I don't eat brilliantly and am very underweight. I don't want to stop BF as I know it's best for my son but I'm loosing weight...??? How can I improve my diet? P.S. he doesn't like chicken!! :(