Hi,
First of all, if he is sleeping through the night he probably needs to feed more frequently during the day, he needs to make up for what normally he would eat during the night. So that sounds normal to me.
15 weeks is also a time of growth spurt, according to books, so he may be just doing that.
Then, if you are supplementing with formula, you are not going to keep up with his demand, sorry but this is how it works, demand=supply. The more formula you introduce, the less milk you will have. If you are planning to nurse for a longer time, eg. for 6-12 months, it would be worthy to start working on increasing your supply. When they are growing older they take more of your milk. A newborn drinks an ounce on average per feed and a 7 month old will drink up to 8 oz. See what I mean?
To increase you milk supply you need probably allow him to nurse more frequently right now, he is probably going through growth spurt, as you noticed and if you limit his nursings now, you will end up supplementing more. It can happen that after 2 or 4 days of this marathon nursing he will be OK and your supply will be up to demand, thus no formula needed. There is a thin line for using formula in a wise way, some women know when not to cross it.
Tips for upping your supply, besides keeping yourself well hydrated, rested :lol: , and eating a balanced diet, would be:
:arrow: use breast compression. this is a technique that helps baby get more milk. It is squeezing breast while baby is feeding. When baby is still sucking but not swallowing put your thumb on one side and your other fingers on the other side of the breast. You should be holding a big part of your breast in your hand, bring your thumb and fingers together and compress the breast.
:arrow: use herbs known for increasing milk supply (you should consult your health provider before) such as Fenugreek and Blessed Thistle, they are can be found in health food stores, they usually come in capsules, take both 3 times a day 1 capsule. They can increase your supply within 24 hours but they don't always work.
:arrow: feed your baby often during a growth spurts