I would definitely recommend adding grain to his daily ration, as an Eventing connoisseur and jumper myself. Plus, grain would help him A LOT. It gives them extra protein, fiber, fat, and energy so that they can maintain their normal and healthy body weight while having the extra energy to be exercised and enjoyed by you!
Hay and good quality grass are very important parts of the horse's diet, but sometimes they need a little extra nutrition. Actually, it is very important for horses to have MORE roughage (hay & grass) than grain/concentrates, because their digestive systems are not made to handle a large amount of grain.
I'd suggest talking to your veterinarian (or any horse veterinarian) and see what kind of grain they would recommend.
Another thing to look into is making sure the horse is completely healthy, and there are no other causes for it possibly losing weight or not performing (riding) like normal, such as worms or digestive problem.
Basically, YES. I really do believe that adding grain would help your new horse perform at his best!
- Plus, ***Always feed by WEIGHT and not QUANTITY***, feed by your horse's weight, WEIGH out the amount of grain you give them (NOT by measuring in "scoops") because different grains are sized and packed differently, so one type of grain's "1 scoop" is NOT the same amount as a different grain's "1 scoop". So, measure the grain out by "pounds" instead, since this is always how the grain labels list out how much to feed your horse. This is VERY important for every horse owner to know and practice when it comes to nutrition. ***
I've included a few websites that you should take a look at, with a few different horse nutrition things on them. The last one is really great about explaining nutrition. Hope this helps!