Usually babies don't increase the amount of breastmilk they drink after a couple months as the milk adapts and increases in calories and relevant nutrients. Does he show signs of still being hungry after a feed or is it just him getting older that makes you think he needs more?
The only way to increase supply is milk removal; putting baby to the breast is the most effective way. What makes you think that you have low supply? Why do you think he needs to increase? Xx