I went thruthis with my now 2yr old, when she was about the same age.
As nother respondant said - there is a growth spurt around 18 months, so keepthat in mind, adn don't expect full night-weaning in a week or 2.
Remember to offer the breasts often thru out the day, 18 months is also a time when they are so busy exploring and learnign and discovering at day time, that they don;t nurse or eat as much - making it up at night. Also try more snacks and a long calm soothign nursing session right before bed.
Explain to him that just liek he goes night -night - your nursies (or whatever you call them.. they were chi-chi's in our house) have to go night night too.
Make them more inaccesable at night - wear a sleepign bra AND a shirt.
when he wakes at night, offer hm cuddles, a sippy of water, sing a song etc. Re-emphasis that the boobss are sleeping and they won't wake up untill the sun gets up.
if he's truly ready then you should be able to cut back to 1 night nursing (try for as far toward morning as you can.. my youngest's DD's last night nursing session to give up was at 5am) fairly well. Adn then transition to no-night nursing as well.
It can be tough. the key is too be gentle but firm and loving. Not taking away something - but replacing it (ie, the cuddles, sippy, hugs, soft singing etc).
Good luck!
And I also highly recomend Dr. Sears's Nightime Parenting :)
~C.