At my barn they give Nutrena Senior bc it has less sugar so they don't get very energetic and Timothy hay. If he's skinny give him a lot of hay and then if that doesn't work buy beet pulp and mix it in with his grain
If you can talk to the horses old owner, you can base it off of thier reccomendations and then if he needs his ration changes because of wieght or work load, you can increase and decrease as you see fit. If you cant get the exact grain, ask for the percentages of fat, fiber and protein. You can find a similar grain that way. Also ask how much hay or forages he is currently given. If he is rather skinny, increase hay intake first. Hay is easier to digest. Also make all feed changes slowly to decrease the likelyhood of colic. Ask the old owner for some of his feed so you can change him over slowly if you care using something new. Most owners are likely to oblige.