How does he vomit? Is it projectile vomiting (ends up a distance from him)? Is it with every feed?
If he has projectile vomiting, AND he is starting to vomit with every feed, I would advise taking him in to the doctor ASAP. While it is more likely to just be reflux, it may also be a condition known as "pyloric stenosis" - don't worry, if it IS this condition, it's easy to diagnose (an ultrasound is required), and also fairly easy to fix. Better yet, the fix is permanent, so no more problems from it!
Pyloric stenosis is where the pyloric muscle (at the bottom of the stomach) is thickened. If it is only slightly thicker than normal, then the condition is often diagnosed as reflux, and whilst it can cause vomiting, it isn't a threat. However, if the muscle is significantly thicker than normal, it can make it impossible for food to go from the stomach to the intestines - which means that no matter how much baby eats, he cannot digest anything, and it can be life threatening. Because the issue is usually somewhere in between those two, most babies don't show symptoms until they are several months old - some of them not until they're starting solids.
With my boy, he suddenly started projectile vomiting at three weeks of age. Prior to that, he was eating like a horse, and gained a LOT of weight. In the week he was vomiting and the GP was attempting to find out what was wrong, he lost all of the gains he'd made over those weeks, and then some. It took the hospital almost a week to diagnose the problem. Once it was diagnosed, he was moved to another hospital, and had surgery the next day. Within two days he was back to feeding normally (he vomited twice in the two weeks after surgery) and he's been doing brilliantly ever since.
I don't want to alarm you, and honestly, the chances that he has this problem is minimal. However, because most doctors don't even think of it, if he DOES have projectile vomiting and vomits at every feed, then it could be worth bringing up as an option - especially since the test for it is not invasive, and is so fast and easy. I had to face I don't know how many nurses insinuating that he was only vomiting because of the way I was feeding my son. The only reason they stopped accusing me of anything was when he started vomiting when THEY fed him.
As a side note: my son did have real issues with wind - more so after the surgery. So, I had to learn to get rid of every last bit of wind in order to stop him from vomiting even months afterward. I did find some over the counter remedies to help with alleviating wind, which made a big difference.
The back arching and stiffness sound like there could be a pain issue involved, so it could be worth checking anyway.
Wind can cause pain, and the general symptoms may be explained with simple wind issues. So, if he isn't losing condition, and if he's difficult to burp, possibly try some anti-wind remedies to see if they help.
If you see any signs of orange colouring in his nappy (it may be so dark it can be mistaken for blood) when he urinates, take him to A&E immediately. It usually means that he is dehydrated and needs medical attention soon. Better to be safe than sorry.
[Edited on 17/05/2008]