Yes, you need protien but it is not where you get your energy from. Fat is your energy source. Changing the protien level 2% isn't going to make a difference. Using veggie oils (corn, veggie) has been known to cause increased inflammation in performance horses and is not recommended. Soybean is better if you are going to use an oil.

You should probably evaluate his diet. He should be gettting 1.5-2% of his body weight per day in food. The majority should be in some type of forage wether that is hay, hay pellets. It also is important that his feed is weighed and if you are feeding a supplement (grain) that it is also fed properly.

There are numerous products that can be feed to your horse to add fat/energy to the diet. Oils, rice bran, commercially made products. I have used a number of these and found that my favorite product is made by Purina and is called Amplify. It is a top dressing to your feed (extruded high fat nugget). It's fat content is 30% which is higher than any other product on the market.

What is his training schedule like, How often is he rode and for how long? What do you feed him and how much?