There is more to acid reflux in an infant besides constant spitting up - some babies are just barfers no matter how much you burp them, but constant PROJECTILE spitting up, regardless of how much you burp your son, is not normal and a sure sign of real acid reflux. Your ped was able to diagnose the condition quickly and easily because there are several tell-tale symptoms that point to only A/R and no other condition - it is not a difficult condition to pinpoint, and because it is quite common and easily controlled with very small doses of baby safe medication, pediatricians are quick to diagnose and offer a cure. No one likes to see their infant in pain. Furthermore, if he were allergic to the formula you give him, he would also be having serious poop, gas, and digestion issues on top of the projectile spitting up (he would be - a milk allergy effects the entire system and would be giving him diarrhea, gas, the whole gamit of digestive issues) - and allergic reaction does NOT just include spitting up.
Unless you want to continue to see your son uncomfortable, give him the liquid Zantac. It is very commonly used, and very safe for the treatment mild to moderate acid reflux in infants.
At 3 months, my daughter was diagnosed with acid reflux. She was presenting all the telltale signs - a constant projectile spitting up, uncontrollable "pain" crying, an inability to tolerate laying flat, wanting to eat and be held all the time. Babies with acid reflux generally cannot sleep lying flat, as it allows the excess stomach acid to creep up the esophagus and causes severe pain and burning - reflux babies are often bad sleepers, or will only sleep in an upright position (in a swing, infant carrier, someones arms). Reflux babies also want to eat constantly, even when they are not hungry, as the lactose in the breastmilk/formula calms the burning of the stomach acid.
Liquid Zantac saved our life, and turned my unhappy infant into a happy, pleasant, well adjusted and pain free love. Try the Zantac for a week or so - if it helps, you know that the diagnosis and treatment were spot on (you do have to give the zantac a few days to take full effect). She grew out of the A/R by 7 months, and we were able to to take her off the zantac altogether.