He looks a little thin but not excessively thin for a pup his age. Mine bother weighed close to 60 at 6 mos but I had a female that weighed 64 as an adult. Like BlackPuppy said, you should feel a covering of meat over his ribs but not see each rib bone. I may have mentioned that you might try making satin balls and feeding them for weight gain. Duke has a lot of catching up to do and he's growing like mad so it will be hard to get him chubby- which you don't want anyway. And with his past he has a lot to make up for.