Have you started solids yet? Could he be reacting to something? Any reflux or anything like that?
He's also at the age where distraction can become a problem - is that the case - is he too busy looking around to nurse or is he just not interested at all?
If it's teeth and his gums are sore, you might try to let him chew on something cold before he nurses - it could help him nurse without any pain.
Is it possible that he's got a touch of a tummy bug (I've found with mine that bfing protects them to the point where they'll get a tiny bit sick, but not too bad.). At any rate, if he is eating somewhat and wetting enough diapers I'd just keep offering and he should go back to his usual intake. If he's not acting all that sick he's probably fine, babies tend to let you know when something is really wrong!