Firstly, while the pics aren't the best, I don't think he looks too thin, but he's obviously not in the best health & is lacking muscle & appears to have back problems(hunter's bump, etc) which would be one reason for atrophy/lack of development. Have you had a bodyworker check him out?
He is routinely dewormed. He is on free choice to hay,water,and salt and on 6 lbs daily of Nutrena Triumph 10/10 (just switched to it) and since I got him, he has been on Nutrena Compete, Purina Omelene 500, and Purina Omelene 200. He has also been on CoMega supreme and Red Cell.
As someone else mentioned, routine deworming is not always *effective* worming. I'd ensure that was taken care of and that if he hasn't had his teeth checked in the last 6-12 months, get the dentist.
How long have you had him & what are the reasons you've changed feeds & given the blood supp? Have you analysed his diet for nutritional balance? Does he get a good nutritional supp to provide what he needs? Ill health/failure to thrive can be about nutrition, not calories.
Considering all the 'high octane' feed he's been on/getting, I'd consider this could be part of his problem & changing to something healthier & lower starch/grain/sugar. It also depends how you feed - eg do you give him that 6lbs over only 1-2 big meals daily, or over a few more small ones? Especially if you can't feed it little & often, I'd avoid those types.
recently attended a Cargill/Nutrena nutrition seminar and they recommended their Triumph 10/10 for him because they thought he was lacking amino acids because of the lack of muscle.
Good on you for trying to get better educated! Call me a cynic, but unfortunately I don't generally think seminars & advice from manufacturers of feed is going to give you an objective point of view though. Particularly manufacturers of cheaper stuff without fixed ingredients. Of course they're going to recommend their own feed. I would suggest you use an independent nutritionist/service, such as feedxl.com or Dr Kellon's site/course to learn more & get unbiassed information.