Hi C.,
I also have a baby now 15 months old. That was and still is the only way to get him to sleep. Try this, This will take a few days, maybe even a week or two, but it will work.
When you normally give him his bottle in his crib start dilluting it just a little at first, then more each night, eventually he will get used to it, and eventually water will be what's left. That's what I did. Just make sure that during the day you put all the formula in to make sure he gets all the vitamins, but you know that.
I hope it helps, and I swear sleeping issues are the most stressful things to deal with. Good luck!
Guess what when my baby was 4 months old I got pregnant again, so I would have loved to have an extra 5 months to not have to deal with another one! You'll do just fine. Just remember to give them all individual time and everything will work out!