It sounds like you're dealing with a nursing strike. I'm going to link you with kellymom's information about strikes, because I think it's the best coverage of this problem that I've seen.
http://www.kellymom.com/bf/concerns/baby/back-to-breast.html
http://www.kellymom.com/bf/weaning/babyselfwean.html
And also LLL:
http://www.llli.org/nb/nbnovdec92p173.html
http://www.llli.org/faq/strike.html
The most important factor, I think, is consistently trying. Most babies who strike eventually go back to nursing avidly. They strike for a whole variety of reasons, and often you'll never know what it was that caused baby to start to refuse the breast.
Lots of women mistake a strike for self-weaning. It usually isn't, especially during the first year.
Have you tried discontinuing all other sucking-- like no bottles, no pacifiers, no sippy cups, etc. Often this does help, since the urge to suck is still so strong at this age. You also might try easing back or even stopping solid food, because breastmilk should still be his primary source of nutrition at this age.
It's also possible that if he's nursing several times during the night, that he might not need so much during the day. The very first thing I'd do I think is to start offering more during the night-- if he's not cosleeping already, take him to bed with you, and sleep with him right against your breast.
Good luck!

Strikes can be so frustrating and upsetting. I think they're harder on the mama than on baby.