The thing that is so great about Platinum is that they have specific supplements for all sorts of problems horses have...and they work! For example, they've got supplements for the horse who is overweight, underweight, has allergies, digestive problems, weak hooves, and the list goes on.
If you're having trouble keeping weight on your eventer, I would use their Equine Healthy Weight supplement. It's a liquid formula that adds those needed calories to a horse's diet. You can either put it over his feed or you can feed it with Platinum Performance Equine Wellness Formula or Platinum Performance CJ. If you can afford going with CJ, I definitely recommend it, especially since your horse is an eventer.
If you're not sure how much to give to your horse, I'd contact their customer service, either online or by phone. Their whole staff has extensive knowledge in horse nutrition and are always so happy to help!