I'm so sorry this is going on, for you and your sons. I'm a single parent as well, raised my older two on my own and now my little guy who is almost 31 months, and it's hard sometimes.
You say "I have read more books that I can count and done a million and one "expert" advice on how to get him to sleep and nothing." But I have to ask, how long were you consistent with any of the methods? I ask not because I'm chastising you, I'm not, but because, particularly at this point, no method is going to work immediately, say in one night. It will likely take weeks or months before he is going to sleep easily and sleep all night every night. Whatever method you choose you have to commit to sticking to it until it works.
Also, since his doctor has checked him out and he is fine, what did the doctor recommend to get him to sleep on his own through the night? If he or she can offer no advice they should be able to refer you to a behavioral/sleep specialist for your son.
It basically sounds as if your son never learned to self-soothe himself to sleep or back to sleep as an infant, so you have to begin at the beginning. You need to understand what you may be doing that is causing this to continue to happen, and change some things around. If he can only sleep when you are holding him, he'll have to unlearn that and to eventually go to sleep in his own. He'll also need to learn that when you come to check on him when he wakes up in the middle of the night that once he's calmed down he needs to put himself to sleep and not run after you. You also need to realize that he more than likely isn't going to like any of it at first, but that doesn't mean you shouldn't do it, you're doing it for him, yourself and his brother.
You may have read this book already but in case you haven't, it offers a no-cry sleep solution for toddlers and preschoolers, not babies:
http://www.amazon.com/exec/obidos/ASIN/0071444912/elizabe...
One idea from the author: "You can invite the "morning fairy" into your house - she is the tooth fairy's sister. She leaves small prizes outside the hall of young children who sleep in their beds all night and don't wake Mommy."
Here's some articles to give you ideas:
http://www.babycenter.com/0_6-major-sleep-mistakes-parent...
http://www.babycenter.com/0_does-your-toddler-have-a-slee...
http://www.babycenter.com/0_bedtime-battles_12321.bc
http://www.babysleepsite.com/toddlers/limit-setting-toddler/
One last thought, think about getting him a Cloud B Nightlight that projects constellations on his ceiling. You can turn it on when you put him down to bed, it stays on for 45 minutes, and if he wakes up in the middle of the night once he's calmed down you can turn it on again as you leave his room. There's different styles to choose from:
http://www.amazon.com/Cloud-Twilight-Constellation-Night-...
I hope you can get this under control and get some sleep, God bless.