I'm surprised he is getting thin that early. My 15-18 yr olds are all fat.
Now my 32 yr old, I just burried him, he looked like a skeleton all 2014. I could feed him till he almost foundered, calf manager, senior diet, red cell, corn oil, molasis, fat supplement, you name it. Straight alfalfa hay, plus choice of grass or coastal, alfalfa pellets.. etc. Just couldn't, keep wait on the ole boy. He had all his teeth and could roll right up untillmthe morning he died. In fact I fed him some extra about midnight one night as I was in the barn. He was 'fine', next morning 7am, gone.
Good luck with yours. I tried every trick I know plus stuff from the feedlot, vet and farrier. We were trying for 33 but missed it by about a week.