The key to weaning and any other major transition is to make sure it is consistent and calm.
I am a co-sleeper...and very dedicated to it and attachment parenting. So, hears what I think might be worth a shot...
It sounds like what my son did at night while he began to teeth, after self weaning himself from the nightfeedings. You may want to invest in Drops or Tablets for teething at night.
But, if he is rooting for the nipples for soothing back to sleep then I would make the nipples less avaiable. Wear a sports bra, or tank bra that covers them and denies him access.
Talk to him about what is happening, and make sure he understands that you are doing this for the both of you. I use the word 'team' a lot with my son, and now that he's older he'll even remind me that we're a 'team'.
You could introduce another form of comfort...a lovey or pacifier. My son had a his 'dino' that we picked out together, and my sister came with us and purchased it for him. We made it a big deal and he loved it. This way at night he would look for the lovey and not Mommy...it took a few weeks though.
There are NO fast cures for this kind of stuff, and using the CIO Method with co-sleepers is kind of counterproductive. Plus, lots of bad information keeps coming out about how it affects brain development and emotional development/attachment moving forward.
The link below is from Dr. Sears a big advocate of attachment parenting...and, great advice for co-sleepers.
http://www.askdrsears.com/html/7/T070800.asp
Good Luck and keep up the good work!
D..