hi lisa - yep, my lo, also 5mths very rarely genuinely cries. when i put him down he always had a whinge and whine, then the fake crying would start and go on and on and on.
i have just started weaning him off the shushing, and surprisingly the 'mantra' has pretty much stopped. i put him down and he has a talk, starts sucking his hands and soon falls asleep. if he does start with the mantra now, he soothes himself out of it within seconds.
do you put him down and leave the room? i mean, is he doing it for your benefit do you think? i've found just recently, the less involved i am with trying to get him to go to sleep, the better he is at it on his own.