You know -you can do amended CIO! He's waking up so much because he wants your attention and comfort and he hasn't learned to self-soothe. I was never a fan of complete CIO, but this is what I did with both of my children, and it worked like a charm:
It sounds like the amended Ferber got him going to bed okay! So, when he wakes (and keep in mind -twice a night is perfectly normal at his age), feed him and change him if needed. Do this TWICE per his feeding schedule. Put him back in his crib. Whenever else he gets up and it's not time to eat or he's not dirty, gently talk to him, reassure him and touch him and walk away. When he cries, give it a few minutes. I'm not talking 10 or 15 or 30 -just a few minutes. If he isn't starting to settle, go in, pick him up and hug him and tell him -"It's time to go back to sleep." Put him back down and if he cries -let it go a little longer -up to 5 minutes. If he seriously is not getting calmed down, go in and pick him up again -repeat. I've never had to do this more than 3 times in the beginning. It's okay if he's fussing -leave him alone! He's just getting himself calmed down. Don't go back in there unless he's full-force screaming and it's escalating! It may take a week or so, but things will start changing. Make sure he has a "lovey." This isn't necessarily a stuffed animal. Many companies, especially Carters, make extremely soft loveys with stuffed animal heads and small blanket bodies or else get a really soft and very small stuffed animal -something too small to keep over his face, but that he can touch to help soothe him. It's incredible how soothing babies and toddlers find stroking a soft item to be! He won't grow up with a terrible attachment either. The lovey slowly fades into the background as the toddler years slip by. The loveys have been invaluable with my kids as well! You may have to listen to a little crying. I'm no fan of the ultimate -let the cry themselves to sleep method, but if you want to stop getting up 6 times a night, you're going to have to let him learn to soothe himself.
Another thing I did with both of mine, who are allergy free and completely healthy, is to start putting rice cereal in their bottles around this age. I breast fed my first longer than my second, but you can mix it with breast milk. People freak out over rice cereal now like they do about everything else they overanalyze to death, but it's fine. Every kid and adult I know has been on it, and it does help them sleep. When it comes to parenting, just remember -if you're completely against something or dead set on doing things a certain way, and it's doing nothing but making everyone miserable -maybe it's time to change!
***In response to your update -NO, 5 months isn't too young to teach self-soothing. Babies who suck their thumbs are self-soothing and my youngest has been doing that since birth. It's not that he's "spoiled" but that he is solely relying on you for all comfort and not realizing that he can provide himself with comfort. Right now he has no idea that anything other than you and your breast can be comforting to him -so he needs to find out that other things are comforting and he can access them. If you leave him alone -he WILL discover a lovey, his pacifier, something that makes him feel calm and more like going to sleep. As long as you rush in every time he cries, he will never have the need to seek anything like that out.