Bzee,
Sounds like you may be going through a growth spurt. Why don't you increase your feeding time to 15 mins per side. Or you can even nurse on demand. The increase in demand will signal your body to produce more milk to meet your baby's needs.
If you decide you'd rather introduce formula, then after breastfeeding you can try giving him an additional 3-4ozs of formula. If you want to continue breastfeeding, than don't replace feeds with formula, otherwise your supply will slowly diminish.
Here's a good article on Nursing and Growth Spurts
http://breastfeed.com/resources/arti...owthspurts.htm