My daughter (3, now, but was 7 weeks premature) had severe acid reflux, and was unable tosuck. We tried EVERYTHING! While she was breastfeeding, she threw it all up so went to formula. Our WIC department was similac at that time, but our dr recommended other formulas. We tried Nutramagen, Alimentum, and finally settled on Enfamil AR. I found that ALL enfamil formulas made her stools a yellowish greeny liquidy sort of thing. I asked the der about and he said it was becuase of the enfamil really being so close to breastmilk. I dont know if you ever breastfed, but breastfed babies poop genereally after each feeding, and its a yellow seedy consistency. While she didnt poop after every feeding with the formula, it ws pretty darn close! My youngest child was the same way, after he stopped breastfeeding, we used enfamil, and his poop stayed a yellowish soft/liquid mess LOL.
Prevacid causes gas in grown ups too:) As long as he doesnt seem to be bothered, dont worry about it. Granted, nobody likes to walk around in a cloud of fecal matter, but if he's happy, dont stress over it. Its not forever and they are only little once. :)
best of luck, darlin:)