I wish my old man looked as good as that! Ours is very skinny and sway backed. He's always been a hard keeper and now despite senior feed and nonstop hay, he still looks like an emaciated rescue case! We are not sure of his exact age, but probably somewhere between 30-35. We've been separating him at feeding time because the others chase him away. But boy, he takes forever to eat now that he can take all the time in the world! He's one horse who thoroughly enjoys his food!
Sounds like your guy needs some supplements. Try adding to his grain: 1 cup rice bran or beet pulp, 1 cup black oil sunflower seeds, 1/3 cup corn oil and a scoop or 2 (tiny scoops) of DAC Bloom (made by Direct Action Company). This is a great way to put & keep weight on senior horses.
He gets beet pulp and it didn't really make a difference. We've tried other things here and there but black oil sunflower seeds are next on my list. The problem is also that he is a picky eater and will eat around things he does not like! When we tried adding oil he just walked away! Took a while to get him to eat the beet pulp but eventually he did.
Try adding all of these things. Switch from beet pulp to rice bran. Mix the supplements in the feed, so he can't eat around them. It's working well for my horse.
Aw! Happy *late* Birthday, Josie!! That's amazing that's she's 30, and she looks so good! Too bad about her eye, but that's crazy that she lived through it!!!