H.B.
Night time nursing is usually the last ones for a child to be comfortable with losing... Is there a specific reason that you want to wean right now if he is still very much into it? If you aren't totally set on weaning immediately, it would be sooooo much easier on your son (and possibly you and hubby too!) to just go with the flow and let his own disinterest develop in time naturally. Sure, you can distract him to reduce a day feed or try weaning away one night feed, etc....but the all-out battles that go along with forcing him to wean before he is naturally ready could be avoided :-) 20 months old is still young, probably too young to really 'understand' the way we do about why he can't do the thing that makes him feel comfortable, full and loved... He doesn't see it any other way, so sometimes I think it's good to think about what they are feeling and going through as you wean away a given nursing time. The WHO still recommends a full 2 years, so it's still very beneficial to him. Every child is different, if he is actively consuming breastmilk each time he wakes at night, then he needs it nutritionally and his metabolism is such that it necessitates eating frequently. If he's not eating fully, then he is still developing emotionally and needs the comfort, closeness and love at night that he so enjoys and has trusted. If you decide that yo want him to replace you with something else for comfort (a blanky, teddy, sippy) then it will be up to you, but I personally like being the one my little one turns to :-) I have read that it is good for babies/toddlers to develop the sense of turning to people rather than things for comfort as young people so that they will turn to people as adults rather than other forms of self comfort that may not be as healthy. You have been a wonderful mother and I know you want the best for your family. Hang in there! Sleep will come again in due time :-) If you are planning to get pregnant again, things will all change for your toddler again anyway.... he may naturally wean due to the change in taste or supply of your milk, then you have avoided the battles and sleepness nights that come with a forced wean.
Have you read the LLL book called 'How Weaning Happens'? Have you considered joining an online Yahoo egroup for those nursing beyond 12 months? They always have awesome pointers and loads of experience. Look under extended nursing or nursing beyond one year....
I wish you the best, hang in there. He will only be little and nursing and needing you at night for such a short time in their life, enjoy it while it lasts.... before long he will be in a new phase and this will be in the past.