Ketones, the by-product of ketoacidosis (caused when your body starts to metabolize its own muscle tissue) usually have a very sweet, almost fruity smell, not metallic.
It's very possible that your dog has impacted anal glands. Many people who have had dogs with anal gland problems have described their dogs as having a distinctly metallic smell. I would make an apointment with the vet to have his anal glands expressed, and afterward, see if the smell goes away. If nothing else, it would provide a good opportunity for the vet to examine him to see if there was anything else going on that might contribute to the smell, but my guess is going to be something to do with the anal glands. Please post back and let us know what happens.
Ghilly
Well, it's really great that you took on this stray/lost dog and gave him a home. Are you sure he's not microchipped, and that his owners might be missing him? If you feel that definitely isn't the case, then fine.
The best thing to do with any new dog, and especially with a dog who was a stray, is take him to be thoroughly checked over at the vet's. You have no idea of his medical history, and can only imagine what he might have gone through when he lived rough.
As for the strange smell....without smelling it, it's hard to tell. Is the smell coming from his breath? If so, then there could be gum disease, or even possibly diabetes.
It is possible this could be what are called "Ketones". This can sometimes develop from malnutrition (the body starts to metabolize its own tissues, and produces what are called "Ketones")
Another thought is he could have slept somewhere chemicals were kept, like a garage or something.....maybe even a bath didn't wash this off?