Hang in there! With three little ones right now, you are doing great! It does sound like he needs to learn how to fall back asleep on his own. You don't mention whether or not he's nursing to sleep for naps or bedtime but if he is, then it's also a problem that he probably doesn't really know how to fall asleep on his own and he's going to need help from you to learn that.
If he doesn't like formula quite yet, then you might want to wait to wean until you have him on a better sleep track. Doing both at once may just be too much for him.
I'd first start with not nursing him to sleep. Nurse him just until drowsy and then put him down. If he cries or wakes up when you do this, then you'll have to create a plan for how you're going to handle helping him learn to go to sleep. Your plan depends in part on your overall parenting beliefs and in part it depends on his personality type. You might be able to shush or pat him to drowsy or if he gets really worked up, you may have to pick him up, calm him, but then put him back down to go to sleep on his own.
At this age, he may really only need to feed 1-2 times a night. You willl need decide which wakings he can feed during and the for all the others, you'll need to let him go back to sleep with whichever method you have chosen. Whatever method or plan you decide to use, the key will be to be consistent with it. This is how he will learn. There may be some struggle from him as he probably won't like the change.
If you'd like help with a sleep coaching plan, you can start here:
http://www.babysleepsite.com/s…..es-part-1/
Or you can consider one-on-one email consults with Nicole.
Good Luck. Let us know how it's going.