I think your horse may need to see a vet. There are a number of reasons why a horse could stay "skinny," and not all of them are lack of sustenance. Parasites, ulcers, vitamin deficiencies, illnesses, and dental problems are just a few possibilities that you may want to consider. Were I in your shoes, I'd talk to a veterinarian. :)
Ruling out an innate health problem, we should look at his diet.
Nutritionally speaking, I'd like to examine his diet in a more detail. How much is "a scoop?" How much hay are you feeding him? It's important to measure feed by weight, not volume. (That means saying "3 lbs" versus "one scoop.") Horses need to eat about 2% of their body weight in forage each day, so assuming your horse should way roughly 1000 lbs, he would need to eat 20 lbs of forage per day. Also, concentrates (that would be your pelleted feeds, beet pulp, etc.) should take up no more than 10% of your horse's diet.
Does he have access to salt? Salt is a staple part of every mammal's diet, and especially that of horses. Whether you choose to provide that through daily electrolytes, free choice loose salt or pressed salt blocks, or rock salt, is really your decision, but it is absolutely necessary.
What breed of horse is he? By "skinny" what do you mean? Do you have any pictures of him that you could provide? Is he up-to-date on his vaccines? What is his deworming schedule like?
We'd love to help you, but need just a wee bit more information! :) Thanks!