Ok thanks :)
As far as I am aware of, Rocky Mountain horses are fairly easy keepers, so you probably won't have much to worry about.
I would feed between 1 to 5 percent of his body weight in good quality hay. The richer the hay, the less you have to feed. So if he's 1,000 pounds, you're probably going to want to feed 15 to 20 pounds, and I would recommend 1st or 2nd cutting orchard, timothy, or a mix of that. A grain hay, such as oat hay, or what is called 3 way (sometimes 4 way) is also an excellent choice, especially in the winter time. I don't know if they offer alfalfa mix on the East Coast, but from what I've heard over there, I wouldn't feed that. Since he is only being used lightly, I would stick to hay for the moment and re-evaluate in a few weeks.
If he's eating all the hay and looking for more, add more hay with each feeding. If he's leaving a lot, then you can cut back. Once you have established how he holds weight and works with just hay, then you can start looking at additional supplements. If you want more weight, you can add from 2 to 5 pounds of soaked beet pulp, which is very popular on the east coast. A feed store employee can show you how to soak it so don't worry about that. Beet pulp is considered a forage and not a grain, is almost sugar free, has a very high fiber content, lots of calcium, and a decent amount of fat.
Honestly, I would start here. See what happens in a few weeks, and if you want to add something to the hay then start independent research and try some of the things suggested on this board. A lot of people recommend vitamin supplements but from everything I've seen, unless the horse is exhibiting a deficiency or being worked hard, he's probably not going to need it.