I agree with the previous poster that there is nothing wrong with a baby not sleeping through the night at 15 months and thank goodness he was rocked and snuggled to sleep with all his medical issues. That isn't "babying" him, its good parenting.
But you also need good sleep and it makes sense to start working on getting him to sleep longer stretchs now that some of his medical issues have resolved. First and foremost, realize that it will be a process and it won't happen overnight. Be committed to the process and recognize that you need to help your son learn good sleep habits - he shouldn't be expected to teach himself.
Start by establishing a consistent and relatively early bedtime. Then decide on a bedtime routine and follow it to the letter every night at the same time (bath, lotion, pjs, stories, kisses/rocking, etc.) For the first week or so, just implement the routine and still rock/walk/whatever him to sleep - he is getting used to the idea that there IS a routine and is learning sleep cues.
The next week or so, follow the routine and rock/walk him until he is very drowsy. Then put him in his bed quietly. Walk out of the room or around the corner or whatever. Give him a minute or two to settle. If he is fussing but winding down, wait and listen. If he immediately pops up and starts screaming, go back in, pick him up again and soothe him (but do not talk to him - keep your eyes closed, the lights off - everything very quiet and not stimulating). Hold him until he calms down and then put him down again.
At 15 months, I think you need to have a hybrid approach. There will likely be some crying/fussing because he is learning a new skill and he'd prefer you put him to sleep the way you always have. But you'd prefer some sleep sometime soon and that is also valid. I would urge you not to let him CIO, though. If you let himi cry, he will probably give up at some point because he realizes it is pointless, but I think its better to communicate to your child that he can rely on you and you will come when he needs you. You can be reassuring and loving while still teaching him to learn new habits.
If he takes a pacifier or any other comfort object, be sure to incorporate that into the bedtime/middle of the night wakings so that he slowly comes to associate that with comfort.
Take it slow. Take naps whenever you can (if you can) so that you are less frustrated and exhausted with the teaching process. This will not last forever, I promise. Hear your baby's cries as an effort to communicate with you and a desire to be close to you (good things!) - not being spoiled or manipulative.
Good luck!
B.
Momma to two sweet children