How long have you had him? He may be grieving his former owner and home, maybe there were other dogs there that he bonded with. It is very hard for some dogs, especially a senior.
The first month will be the worst. Just be extra nice to him but give him his space, hopefully he’ll come around.
You can presoak the kibble in water overnight in the fridg and then add a little plain homemade chicken broth (no onions) or mix with a soft topper. If you don’t see him drinking water, add a little to his food, senior small breeds are vulnerable to get bladder stones, struvite and urinary tract infections if they don’t drink enough water. Take him out frequently to urinate.
Keep his diet simple, maybe a limited ingredient food, I like Nutrisca. I wouldn’t add vegetables for now, they can cause loose stools in some dogs. I wouldn’t add any supplements unless advised to do so by a veterinarian that has examined him.
