Could you try taking the boobie out of his mouth, JUST before he falls asleep. If he stirs, pop it back in, wait a bit and try again. Keep trying till he drops off without boob in his mouth. Then slowly take it out earlier and earlier till he's used to not having it. It's described better in Elizabeth Pantley's "No Cry Sleep Solution". I tried it to get rid of the dummy and it worked quite well. (We still have the dummy, but that's because of other issues!). It's not quick, but it is gentle!

If, like Janno27 says, he likes the movement of the sling, perhaps a hammock would work for him? You can hire them to try before you buy. That might also be good fom the point of view of keeping him away from the smell of your milk - maybe that's why he wants it?

Good luck!