I use the lead as a casual tool, a case of we're going for walks so somethings going to happen. It can happen for a number of reasons though: playfulness, excitement, disregard for you, dislike of the lead, a desire to be let off ... its really hard to jugde but try not to make a big deal of going for a walk (excitement wise). Next time you do if the biting occurs drop the lead and walk off (in you garden) and have a sit down for ten mins, as a usual they will loose interest pretty quickly if they are not getting attention - go back to the lead collect it and take it into the house, do this everytime he acts like this - 4 or 5 times he should get the idea and when it gets to the 5th or so time correct calmly.
It may be an idea to buy a cheap lead you dont mind getting chewed - its really a matter of desensitising him to it