I agree--cut back on his grain. Try rice bran pellets. I had an Arab who looked just like your horse does. Not exactly what you'd call 'skinny,' but his hip bones looked like your horse's do, and I was desperate to get a bit of weight on him, but until he was 6 or 7, it just didn't happen despite my best efforts. That was some time ago, however, and I was not aware of the benefits of rice bran (versus grain which as other posters have pointed out is hard on the hindgut) for weight gain. Oil is also good, but my horse like yours was picky and wouldn't eat it. Bet he'd LOVE rice bran pellets. It does come in a bran, too, but I have found the pellets are generally more palatable to most horses. Let us know how he does--he is GORGEOUS and very healthy looking. Better a little thin, than too, fat, too!
He gets rice bran pellets now it makes him all shiny and like really SLEAK but he's not gaining any weight i'm keeping him on it though just because he looks like a new penny and feels like silk. Are you familiar with natural glo? It's all phosphorus calcium balanced and pelleted and he adores it so I can feed it to him without worrying. I'll let you guys know how he is doing we are going to his first show this weekend! I'm all excited even though it's just a little schooling hunter jumper show down the street.
I used to feed Natural Glo. Yes, it is rice bran, but it's been processed more and has other stuff added to it. Have you tried just generic rice bran? If you're just using it for weight gain, then you don't even need to be paying for stabilized rice bran. Might save yourself some money and give your boy just what he needs.
He won't eat beet pulp, we tried and he ate it for a little while then stopped. He is very picky and he's on probiotics to help digestion I don't know if I mention that earlier but yeah anyways he is gaining a little bit of weight by some miracle, I don't know how I havnt changed anything. Maybe because he just lost his last baby tooth? He is still underweight in my opinion though its not normal for a 16.3 hand horse to weigh just over 900 lbs