HI--
Both my kids have food allergies, so I can relate. A few things. First, if he's on prevacid then he is reacting to something he is eating still, or he has what's called a hiatal hernia. This is when the stomach gets up under the diaphragm and causes symptoms of acid reflux. It is a simple chiropractic adjustment to release the hernia (it's a common problem in infants). I could even talk you through how to do it yourself.
Next, no one ever "outgrows" an intolerance to milk. All babies are born with the ability to digest lactose (the milk sugar) because even breast milk has lactose. The ability to digest lactose lasts through toddlerhood (when most mammals wean from breast milk). That is why so many people are lactose intolerant--we aren't meant to drink milk past our toddler years. If a child has a dairy intolerance as an infant, then it is an intolerance to the milk protein (casein and/or whey). This doesn't change as someone grows. However, the symptoms of the intolerance do change, which is why some people think their child has "outgrown" the intolerance. For example, I was really allergic to milk as an infant and small child. I would throw up every time I ate it. As I got older I stopped throwing up, so we thought I could eat it. However, I was diagnosed with irritable bowel syndrome and had problems with diarrhea for years. Turns out my irritable bowel syndrome was simply my dairy allergy with different symptoms. No more dairy, no more IBS.
So, bottom line, you probably aren't going to be able to switch his formula, and will have to steer clear of dairy always. I know a lot of people will disagree with what I've just written. Not only have I experienced this, but have seen it repeatedly with people I have assisted with food allergy issues (after both my children had lots of problems I've become something of an expert on this subject).
Good luck!
J.