my nephews got his first sparring session 2moro (abit short notice but o well) anyways he is 16 years old and he is shorter than everyone in his weight division (10 stone) because of his toned muscles anyways he has to fight guys who he goes upto their chins or probably shorter, i have decided to teach him bobbing and weaving but not exactly sure how to, any tips or advice would be helpful thanks

Your head pattern follows your knee bends,hip bends and your footwork.Through this he will become hard to hit and will create the correct angles while maintaining his prefered fighting distance. Get his head doing U shape ups and downs from leg movments so he practices moving his head from inside the guard the outside of the opponents guard looking for the openings and angles both gloves up at temples when in that close,body bent and sucked back knees bent and doing the work. Then bring his footwork in to the whole thing so he can create those devestaing angles from the outside wen you can really cut loose as the opposition adjusts.
So when youre shorter in reach and want the close in fight to suit your added lower centre of gravity and added leverage in shots off your legs ..then: you train for flexability in your joints, hips, knees especially ,stretches so you gain fluidity on the move ,get him to bend at the knees and train him to move out to the sides on angles so you introduce the footwork with the joint bends .Watch how Mike Tyson was origionally taught to do this and copy his every move.

Oh experiment too ,heel to toe on the lead foot and shuffle up the rear at times. Other times on toes with heels simultanousley lifting and lowerring creates openings and avoidance in head movment too.The trick is linking them all up into movments that are pre thought out techniques.Swithing feet as you go under the opponents arm and then spinning the rear foot around the back of the lead foot is another great opening creator and a massive leverage move for a bomb shot or outside hook over the arm etc as they adjust to follow you.(that is safe to switch there as your away from his other rear arm and moving further away as you do it so you only have one arm to deal with and thats the one you went under.This is made for short stocky blokes who can get in low around larger mens elbow points disrupting them forcing them to always want more space.

do you know any helpful websites?

Its pretty hard to explain really, its like dancing, I can offer this (pretty much what Andre said) it dosn't come from the neck or the waist or your lower back, it comes from your toes knees and shoulders then your neck.

Just make sure the kid knows basic foot work, relaxes, gets on his toes and down on his knees slightly, then just pray he can dance :P

Bobbing and weaving begins with a slip; for example you slip inside a left lead.
Do it this way; slide your left foot forward and to the left, placing your wt over the left leg. Then slide your rt foot to the rt and put your wt on the rt foot; thisb should carry your wt low and to the rt. Or. as he leads a left, drop wt to your rt leg, let his left go over rt shoulder, step left w/left ft, slide body over it--- this should keep body low.

This is basic as hell- pm me and I'll run it down in detail.

what do you think of this....

someone told me to tie a string from one end to another and let the kid bob and weave through it

Its good to perfect your rolling and foot work, it dosn't really help you to develop it from scratch.

Start at the foundations; become flexable and mobile, sit down and do the splits, slow stretches then get your head down to your knee while you hold the stretch,lay down on your leg. and do slow stretching squats too. Standing ,hold your heels and pull down so your head touches knees etc.
To break it up ..Basket ball moves help alot and tranfer to the ring well, table tennis too, anythng that involves hand eye coordination and continual movment with joints bent .
This will lossen you up so you can bend low and then introduce footwork so you can move low and stay (on balance) while weaving around and going under shots, yet still looking up hunting for the openings and evading at the same time. Practice in stages then bring them al together for him.