My GSD puppy keeps chasing and biting his tail and we can't make him stop. He gets LOTS of exercise and has an older dog to play with. We give him Kongs and Raggers to play with but it doesn't seem to stop him. We tried a tiny bit of vinegar on his tail but he licked it off we then tried lemon juice but he did the same. We also tried distracting him with chews and other toys. He either does it at home or whilst walking along the street, we are constantly correcting him. He is not nervous or anxious but a happy, confident, boisterous 14 week old chap. How can we stop this behaviour?

When he bites it does he keep biting it as though it itches? Have you examined the skin carefully?

Try bitter yuck. It tastes pretty bad.

Might want to see if this is something neurologically. Your pup could also be Obsessive/Compulsive. I think you should talk to a vet and/or trainer if its as bad as your saying.

Just stick to correcting it and redirecting his attention to something else. 3.5 month old puppies shouldn't have deveolped any bad habits at this point.. Which is why I would direct you to your vet for some advise or even to a trainer for some insight by actually seeing the puppy do it.

When he 'bites' his tail it's more a case of holding on to it for a few seconds. There's no nibbling or chewing as such. I've examined his tail and I can't see any skin abrasions or dandruff. It looks like a healthy tail. He's been vaccinated and is up to date with all his worming medications from the vets. So I'm concerned it is OCD in one so young and how to stop it. I appreciate everyone's help and advice,
Thank-you.

Luckys suggestion of the bitter spray is a good one. You need to make him not want to chase his tail at all and by making it taste awful this will help.
You can also try distractions... whenever he goes to chase his tail call him over to you and offer him a toy.
You can also use this as an opportunity to teach him the command "come".

Brody one of my poodles chases his tail whenever he gets clipped.
During winter I leave a lot of hair on him for warmth, but when the weather heats up I strip all the hair off him so we can go to the beach without him dragging home tonnes of sand.
Well, when I clip his tail it goes from a giant puff ball to a skinny 'stick'.
That skinny stick is very appealing and he chases it constantly for a few days. I just tell him 'no' when ever he starts and distract him. It usually ceases after a few days.

Thanks for that shazamataz, we've been trying to distract him with toys etc. when we're at home, but it's most difficult when we're walking in the street and he keeps whipping round to grab his tail. I'm not sure what to do about that.

If he does it on the street I would just ignore it. Jerk the leash and increase your pace for a few step so he has to speed up with you.
Once he has forgotten about his tail, slow back down to a walk again.

Ge is only young, I think he will grow out of it eventually, it just might take a bit of effort and a few different techniques to help him along.