no real "cure-all"
Hi - I've been there - well, not exactly the same problem, but I have a 2 year old daughter who has gone through phases where we've done things we shouldn't just to get her to sleep. Sometimes you have to do that to get some sleep yourselves.
My advice to you isn't pleasant. I think you should pick a time when everyone is healthy and after some good sleep - then start boot camp. Maybe start slowly - I don't know. But start with a good bedtime routine by 7:30/8 - bath, books, whatever. Quiet time. No movies. It will be hard and cause problems most likely. But you have to do it. Be consistent. Also, constantly be preparing him for bed by saying "after bath we'll read books and it's time for bed" or something like that. I have to do that with my daughter or we'd read every book in her room! Earlier bedtimes can lead to smoother falling asleep. Try to have at least 4-5 hours between waking from nap and bedtime so he's tired. THen I think bedtime routine by 7:30-7:45 will help - then hopefully he'll be asleep by 8:30. Differs for every child, though - so whatever works for you.
Be gentle but firm - no nonsense type of voice. AFter books or whatever you do, put him in his bed, say I love you and good night, then leave without lingering or looking back. He'll cry, scream, throw everything in the book at you to try to get you back, but you have to stay firm. Your older daughter will just have to sleep maybe somewhere else or stay out of her room for this part - whatever is necessary. (You say it's not fair to her but it's also not fair to your son to be learning bad sleep habits. Your daughter can bounce back better - should be going to bed later anyway than him.) Go back in every few minutes or so and repeat the goodnight message, but don't pick him up/hold him.
Now this may not work for you - I'm just saying it works for us. My daughter is so blasted stubborn and knows every trick in the book to get us to come back into her room. But after our routine and good night, we leave. Then if she keeps it up, one of us just stands outside her room so she can't see us and tells her it's time to sleep, etc. SHe gets it eventually.
I'm no expert - and I know others do not agree with letting your child fuss or cry it out or whatever, but I am a firm believer in doing what you can to teach your child to fall asleep on their own - it's a healthy habit for them.
Have you tried the birth clubs on Babycenter.com? Wonderful advice there! I don't know if I've helped in any way or just rambled, but know that you're not alone! BEst of luck to you - keep at it and it will work eventually (like potty training!)