I can relate on this issue....DS5 has never been a good sleeper. For literally - NO exaggeration- the first 3 years of his life, he woke up an average of 4-6 times a night. Both my husband and I were seriously sleep deprived. Just in the last year or so, he has more nights a week where he will only be up once a night and 1 or so nights a week where he sleeps through the night. He sleeps in his own bedroom and due to his Sensory Processing Disorder, hears every little sound - he is terrified of the air conditioner kicking on and off and some nights refuses to do his teeth and potty because he is scared to death that the noise will kick on while he is in the bathroom (and our air is NOT loud - it's actually a brand new outside unit as well - he just has super sensitive hearing). He has a sound machine that we turn up full blast at night to try to help drown out the noise and when he was a baby he would fall asleep to our vacuum (so we taped it on a recorder and had it on constant repeat).
He had GERD very badly when he was an infant and pretty much would only fall asleep on DH or my chest and a handful of times in his car seat...i would strap on his little baby harness and walk the neighborhood at 2 and 3 am trying to get him to sleep...and of course the infamous hour long car rides through the area that never seemed to work.
When he turned 4, we had to make a special chart for him where I photographed him completing his night rituals (bath, teeth, story....) and when he finished one step, he would turn the card over, for each night he completed all the bedtime routines he would get a sticker on his chart and once a line was filled up on his chart, he got to buy a new book at the store...that cut our finally getting into bed deal down to 30 minutes instead of an hour. We read 4 books every night and then DH and I take turns each night laying next to him until he falls asleep....but I don't know if he will ever be a sleeper.