purified water for baby bottles

My son Sam is going to be 4 years old this March.  I nursed him as long as I could (it only lasted a couple of months), and I felt so guilty about switching him to formula.  He had digestion issues with everything - breast milk to Soy to every formula out there.  I ended up sticking with Nestle's Good Start powdered formula and mixed it with bottled water and it solved the digestion problem.  It was always a hassle to be on-to-go and needing to have a clean bottle liner, sterilized nipple for the bottle, and then have bottled water with me at all times that was room temp