I HAVE AN ALSATION PUP, 12 WEEKS, HE IS NOT EATING MUCH AT ALL, AT FIRST HE ATE FINE, BUT THEN HE STARTED TO LOSE WEIGHT, AT FIRST I THOUGHT HE WAS CONSTIPATED AS HE WAS ARCHING HIS BACK UP, VERY THIN AT THE BACK END, I CHECKED HIS POO'S AND THERE WAS A FEW BITS OF SHAVINGS (STRAW, SAW DUST), HE IS HAPPY ENOUGH,  BOUNCING ABOUT FULL OF BEANS, HE HAS ALSO BEEN WORMED, ANY IDEAS

a pup that young that isnt eating should probably be taken to the vets for a check up, they can easily become dehydrated and underweight very quickly due to their small size. does he have access to shavings to eat them?

I would take him to the vet asap if he is eating nothing at all during the day, pups can lose condition very quickly and it is better to be on the safe side. Is he still drinking?
Could I politely advise when you post not to post in capitals please, it's classed as shouting. Thanks
I do hope nothing serious is wrong with your lad and this is a temporary blip

Get him to the vet as soon as possible.If he has eaten shavings/straw he may have a blockage (life threatening) What product was used to worm him as not all products cover all types of worm. Don't know if this could be the case but Tapeworms shed segments in poop & around anus that can look like straw?  If your dog has eaten cat poop, raw pork or vermin/poop or has had fleas then he may have tapeworm. But a vets trip is necessary either way.
Angela

Any pup that looses weight and stops eating should be taken to the vets ASAP.
Pups are dreadful at swollening things. Not evrythink can be digested or absorbed or passed.
I hope he hasn't got a blockage. I would get him checked just to be on the safe side.