I don't know if this will help, but anything is worth a try. My son ALWAYS had trouble sleeping on his own - literally fron day one. He was 17 months old before we had his eyes checked and his vision was -8.75. Turned out the problem was that when we put him down, he thought he was alone and he was scared. After we realized what the problem was, I was always grateful that I gave him what he seemed to need - human contact - when he needed it.
Have you ever had your son's eyes checked by a pediatric opthamologist? Pediatricians won't tell you, but I found out because of my son's poor vision that parents are supposed to have their baby's eyes checked by an opthamologist by the time they are six months old. I had NEVER heard that.
It's just a suggestion and may be the wrong one, but I know that when you are at your wit's end, you are willing to try anything. Been there!
One more thing. Invest in a box fan. Put it near the bed, but facing away and turn it on high when you put him to bed. The white noise scrambles some brain waves or something and actually seems to encourage sleep! It also drowns out little noises like the television in another room or the house creaking. Try it - it's worth the $10. In fact, buy two box fans because you and your husband will want one in your room too!