S.H.
My almost 15 month old daughter has been going through this the last maybe 2 months, but seems to be getting better. Has your son become more active and mobile lately? My daughter started her phase after she got pretty good at walking. She went from being sooo easy to put to bed, to screaming as you say your son does. I used to be able to tell around 8:00 that she was getting sleepy, lay her down in bed, say goodnight, & walk out. She would still be awake, but not fuss & would go to sleep on her own. Then all of a sudden, she would scream when we tried to rock her or put her in bed! We would struggle with her until sometimes 11:00 before she would finally go to sleep. After about a month of this, we started putting her in bed at 8:00 and would shut her gate on the door. She would get out of bed (toddler bed) and cry. We would wait 10-15 mins, then if she was still crying we would go put her back in bed, calm her down (without holding her!), then leave again, repeating the 10-15 mins if she still cried. After a few nights of this, she crawled back into bed and went to sleep after only 5 minutes!
So, maybe your son is just in that phase. Perhaps you can try the 10-15 mins rule. I assume he is sleeping in a crib, so when you go check on him after 10-15 mins, just lay him back down, cover him up, make sure he has his pacifier/bear/etc. I would talk to my daughter and rub my hand through her hair, and sometimes sing to her to calm her down. Good luck!!
Edit-- I just read the other response. I personally don't agree with finding some sound to help him go to sleep. My parents did the fan thing with us and we always had to carry a little fan with us if we stayed somewhere else! You can get so used to the sound that you have to have it all the time! Same goes for complete silence though too. My dad has to have the room quiet and dark for him to sleep...