Hi, K.! I, too, have a 16 week old. She has become more active than ever! In fact, when we put her in a pack n play last weekend, we walked in to find that she had moved 90 degrees!
If your son is arching his back and crying a lot, than he is probably having tummy trouble. My daughter was miserable her first 6 weeks -- arching her back, revving up and crying for hours, wouldn't sleep at all! After six weeks, my husband saw blood in her diaper -- we were alarmed, to say the least. After a frantic call to the doctor and reading Baby 411, we realized that she had an allergic reaction (to cow's milk). We switched to soy formula and now, feeding time -- besides the wriggling around -- is much easier on all of us!
Also, my doctor (who I really like) said that the gas drops are ineffective. If your baby has acid reflux (like mine), the sphincter to the stomach may not be fully developed and gas is getting trapped. I think you would have notice this the first weeks, so I don't think his discomfort is gas. If you think it is, you may want to try Tylenol (ask your doctor first) as this will ease the pain. This is what my doctor prescribed -- and said to throw out the gas drops as they are a waste of money (although some of my friends completely disagree). In our case, the gas drops did nothing since her case was severe (she is now on medication for acid reflux -- and the volatile spitting up has subsided).
Also, our little one sleeps like a champ (first weeks, she wasn't able to sleep much)! Now, at the first notice of sleepiness (yawn, red eyes, etc.) we put her down and sing her songs or talk to her about the day until she is settled in and then we let her fall asleep on her own. Almost always, she is fully sleeping in 10 minutes without much fuss. If we wait too long, she can't calm herself down and fights going to sleep -- in these cases she ends up moving so much she misses her entire nap (which makes the following playtime less fun).
I hope this is helpful and that your little boy feels better soon (and you get some sleep)!!