My 10 month old golden retriever has stopped eating and has become very sluggish. He has lost weight, you can see his ribs, backbone and hips. What could be causing this?

Any of a number of things needing a vet's attention, see My dog won't eat
https://www.askmehelpdesk.com/dogs/information-articles-our-dogs-expert-labman-53153.html#post254171

cbreeding, please follow labman's advice and make an appointment with your vet ASAP. I find it very worrisome that a 10 month old is refusing to eat and is losing weight. Without being able to examine your dog online, it is impossible for anyone here to tell you definitively what is going on with your dog.