Try to think about what changes might have occured in the past few weeks, even subtle ones.
If it is teething, I highly reccomend teething tablets. Target and CVS sells them now. They are homeopathic, docs don't believe in them but don't condemn them either. Trust me- it's a LIFE SAVER. Not only did it drastically reduce my baby's pain and discomfort, it has some chamomile which helps them get and stay asleep. He could also have acidity in his tummy, try GRIPE WATER, also a product that Target sells now. Also homeopathec. And equal response from the docs. Basically, that one calms any stomach upsets that they might have- even if it's as mild as hiccups. I only use the above as needed, but man are they a must have!
If he's a little one, maybe try swaddling him again? He might enjoy the tightness and feel more secure that way. Also, First Years makes a device that you put under his mattress that will vibrate it (remote controlled). I guess that might help him if it's that he got used to the motion of the swing.
I promise you that if you try the above (to eliminate possibility of discomfort and increase his comfort) that he WILL adjust. Do not give into the temptation to cut corners just to get some sleep. I know it's easier said then done, I'm also a FT working mom, but it pays in the long run. My baby is now 1 year old and has gone through her bouts of giving me a hard time (slash NIGHTMARE) but I stuck with it and now she is Ms. Independent about going to sleep and even comforting herself to go back to sleep. We went from times where it literally took an hour to put her down to now it takes under a minute. There is hope, good luck! -N