Well, you're probably just going to have to stop feeding him and explain that there are no feeds until after 6 am. (I'd start with 6 am and if you get that going, then you can move it to 6:30 am or later).
One method that many mom's have found successful with breaking EW habits with toddlers is the 'sunrise clock.' This is a clock that simulates a sunrise by having a light that slowly gets brighter from the time you set it. So, you can teach your son that when the light on the clock goes on, then he can have a BF. But before that he will NOT be getting a BF. Of course, he's going to test you!!! he's probably going to make a right ruckus! Perhaps you could have DD sleep at grandma's for a couple of nights, or else you can just tackle this on a weekend so everyone isn't too tired when he yells the house down!
You can try a reward chart as well. 18 months is a bit young, but I did one with my son at that age that worked pretty well. He would get a sticker every morning he waits until after 6 am to wake you. You can even explain to him that it's OK with you if he wakes up as long as he plays quietly in his room. I don't know if he's in a crib or a bed, but if he's in a bed you might want to allow him to get up to look at books or put some kind of quiet toy in his room. The thing is that once the BF is taken away and he gets up early a few mornings to play, that will probably get old and he'll start sleeping later 'cuz why bother?
The best thing is to have SOME kind of visual cue for him to know when it's OK to get up. Another thing mom's have done is put stickers on a clock face to show when wake-up time is -- so you'd put a sticker at the 12 and 6 positions. You'd show him that when the hands are pointing to stickers that then it is OK to get mommy for a BF. Otherwise he needs to play quietly in his room. If he fusses, you are allowed to go in and say "Honey, it is too early for BF. Mommy's boobies are sleeping. When (clock light turn son; hands point to stickers) you can get me for BF. Until then I need you to play quietly. I love you!"
Like I said, he's going to test this as it won't make him happy! But if you hold your ground AND reward him for complying, then I think you'll be pleasantly surprised!! In fact, you may want to give him a small prize when he successfully complies. At his age, you might give a small stuffed toy or whatever. And also praise what a BIG BOY he is. REally brag about it to family members all day if you can get him to successfully wait until 6 am. The first time you try this, I'd probably reward him as long as you don't feed him until after 6 am even if he shouts the house down. That will set in his mind that waiting for BF has a reward attached. Then you'll explain that next time he only gets prize if he can play quiet without waking you up.
