I'd say he's underfed. I'd get a fecal done for parasites though first.
My blood is a approximately a year and a half old to the best of my knowledge. And he's 7 pounds or 3200 grams. at a year old he was outgrowing a 41qt. Lillyorchid (allison) had him in it. And so that's what I did. But he was ready for something bigger.
He gets a medium rat once a week. Or 3, 60 gram small rats at this point. Eats every week. And not obese like a lot of bloods you see.
I'd check yours for parasites. And then start giving him 1 small rat a week. A prominent backbone is a sign of underfeeding and or parasites.
Best of luck to you! And I hope this helps.
Hope this helps.