D.C.
There is a lot going on here, but I've had a very similar baby - reflux along with a milk protein allergy that caused all the symptoms you mention above.
It sounds to me like his formula still isn't agreeing with him. I don't know which you are using, but you might talk to your ped about a different one. The key here is the explosive bm's. That has nothing to do with reflux and everything to do with a food intolerance. Hypoallergenic formulas include nutramigen (made by Enfamil), alimentum (made by Similac), and, if those aren't tolerated, there are some prescription ones that your ped can have you try.
For the reflux, the best advice for this is to give babies half as much food per feeding, but feed twice as often. The more the baby has at one time, the more can come up at one time. So, for example, try giving just 2 ounces every 2 hours, instead of 3 or 4 ounces every 4 hours.
You are doing all the right things with keeping him upright. My reflux baby loved being in a moby wrap or a ring sling. It kept him upright, and my hands free.
I tried rice cereal for the reflux at about 4 months. It did not do anything to help at all. And, made bm's more difficult. I do not recommend it.
For future reference, keep an eye on his weight. Every time he goes through a growth spurt, you will need to readjust his prevacid dose or he will become miserable again.
Good luck!