I assume all the formula changes have been with the ped's supervision. That said, the spitting up and arching back is not from lack of burping. It's classic gerd (reflux) symptoms. Reflux is really tough. Some babies with mild reflux do well with the sitting up after eating, small meals every few hours, etc. So, you are doing exactly the right things for reflux. Unfortunately, some babies don't really respond to this stuff (keep doing it, but he don't be surprised if he still has reflux symptoms). My second baby had reflux that didn't respond. I did all these things and medication and more, and he still spit up after every feeding and had trouble sleeping because the reflux would act up every time I laid him down. Almost all babies do grow out of reflux eventually, but it can take a long long time - anywhere from 4-12 months or more.
Obviously keep talking with your ped. But keep in mind that in general, pediatrician's don't get very concerned about reflux. It doesn't cause long term problems and babies grow out of it. It's really REALLY hard on the parent with all the spitting up and lack of sleep - but from a ped's point of view, as long as the baby is growing properly, it's not a long term health problem for the baby.
One lifesaver for me was baby wearing. The easiest to ways are a baby Bjorn or a Snugli. I also learned to do a Moby wrap. These are GREAT for reflux babies and their moms because you can wear him, hands free, and it holds him completely vertical which helps to keep the formula down. When the reflux was at it's worst, the ONLY way my baby could sleep was when I was wearing him. I wore him all day long sometimes - around the house, in the grocery store, etc.
Good luck to you. Being a mom to a reflux baby is very very hard. If you have help available (husband, family, trusted friend) who offers to help, take them up on it and SLEEP. The inability to sleep for any length of time was the hardest part for me.