One thing to consider in this photo, is that you've got a dark-coloured dog sitting in front of dark-coloured clothing... right there you're losing contrast in both luminosity and colour. If possible, give him some space of his own in the composition against a lighter background and he'll instantly gain some visibility in the photo.
To get any kind of detail in him though, you're going to have to get your flash off to one side. This will help to create shadows and highlights that show his features. If you can't do this, perhaps throw a white towel down on the ground in front of him so it reflects a bit of light back up under his body to fill the deep shadow. This won't completely fix the lack of definition but it should help him to not be a black hole of light!
Another tip - I assume the sweater on the right side of the photo is supposed to be white? You're under-exposed by about a stop, maybe a stop and a half. If you make that white look white, that'll help Mr. Dog be a bit brighter and less muddy in the shadows and give him a bit more contrast

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