Increasing milk supply

Help! My baby is 4 weeks old and I'm not sure hes getting enough from me when breast feeding what can I do to increase my mill supply, I let him feed as long as he wants and feed when he wants it, but after about 10 minutes he starts crying and comes of the breast then back on and keeps repeating, or hell feed a little longer and falls asleep the wakes up crying because hes hungry, I drink plenty of water due to suffering with alot of utis last year and I also express and pump inbetween feed aswell but I feel as if hes not getting enough and it's really getting me down :(