Dewclaws can be a trait in mammals, birds, and reptiles, but to the best of my knowledge such animals do not have opposable thumbs, such structures being a signature trait of primates. The dewclaw is a vestigal appendage which only typically makes contact with the ground when the animal is running, and not when standing. They truthfully are not generally that useful, and in domestic dogs can sometimes be surgically removed by a veterinarian, depending upon local laws and other circumstances. A closer equivalent would be if he had a small vestigal claw on his wrist or ankle, I believe, although a dewclaw is probably the closest thing that the animal species in question have to a thumb. Regardless, renaming him Dewclaw for the hell of it would be hilarious!