Is he being regularly dewormed on a rotational schedule? I agree that you should feed by weight and not by quantity. I'm not familiar with Nutrena SafeChoice, but make sure that "2 scoops" is the amount recommended for a horse his size. Another option would be to switch him to a higher protein feed, like a senior feed. Senior feed has been my go-to for a horse that just needs those extra pounds.
There are all sorts of supplements to look into – I don't want to start recommending them since I don't know much about your horse. SmartPak is a really great place to get them. If you go to their website (SmartPakEquine.com) you can take a quiz that can help point you in the right direction for what your horse needs. The costumer service is great, the products are great and reasonably priced, and I am a happy costumer!!
In addition, make sure that he has free choice salt. In the pastures, with multiple horses, it's a good idea to purchase two 50lb salt blocks (a white salt and a mineral lick), so the horses have all they need and can pick and choose between the two to get what they need that day. I am a huge advocate for Himalayan salt licks and have had great results with them! The horses can't bite off chunks and they don't dissolve in the rain!
Is he getting his monthly dose of psyllium? If he's not, he could be sandy. Make sure he's getting SandClear or another form of psyllium once a month for one week.
He's not underweight, but is almost on that line. He is a little bit lacking of muscle over his topline, which is contributing to the "thin" appearance. Building that up through collection work under saddle and lounging with side reins and a surcingle (only do this if you know the proper adjustment of these training aids with the help of a trainer) will help.
Good luck, and keep us updated!! :)
Never underestimate the power of human stupidity. – Robert A. Heinlein