Have the vet check him. Look at his head , under his ears, sheath, on the tail top and bottom, and rectum.
There are grey horses that are okay,the grey quarter horse who is 10 or 12 has no tumors, the ones I have owned that had problems were TB and Arabian. The Arab mare was full of tumors , found after she was euthed , necropsy done by the Vet ( he asked said he wanted to see) her tail was covered in tumors, that would crack and bleed.
He is a nice looking horse. He does need a few more groceries. My TB's that had been starved and brought back never did get a filled in top line, but they were older horses.