Hi JB,
My son is 3 months old too and also has acid reflux. I was feeding him 3 ounces max and he would be satisfied for 3 hours. This week he started asking for more so now I give him 4 ounces during the day time feedings and he seems very content. At night he is still getting 3 ounces at a time because of the reflux. Sleeping with the stomach working at full capacity is never good for a baby with reflux. Try increasing the volume slowly and see how it goes. Soy milk is best and keep him on an upright position for at least 20 min after each feeding, including the night feedings. I also recommend the Tucker Sling sleep system, my son sleeps much better on it. It helps A LOT with the reflux and sometimes he even sleeps through the night. And of course, babies should always sleep on their back unless recommended otherwise by their ped.