I noticed the times his night nursings increase are when things are stressful he needs you to help him settle back into his life again - away from home, uncomfortable with teething, sick with a cold. You are his mama and he is not using you; he needs you.
A suggestion is to bring him to bed with you when he wants to nurse or put his crib in your room. It is SO much easier to meet his night time needs when he is close to you - you barely need to wake up!
If you do choose to wean him from night nursings, do so gently. Hold him and rock him and let him know that you are still there for him, but your breasts are not.
Here are 2 great articles on the subject:
http://www.askdrsears.com/faq/sl1.asp
http://www.askdrsears.com/html/7/T070300.asp
Good luck! Enjoy that baby! :)