Similac Isomil. Used to use Gerber Soy but he started to refuse to drink it so we switched. He's not able to tolerate milk (gets a rash, mucousy green poops, lots of painful gas). We get it at aWalgreens, Harris Teeter, or on Amazon (I love the little travel powder pouches and 8 oz ready to feeds for when we're out). It is harder to find so we also keep an extra box on hand at all times.
Re: moms who use soy formula