First get a good protine/ fat feed for him. You can also supplment with flax seed, beet pellets etc. He may not be as under weight as you think if he is as young as you say & is still growing. Also a higher protine hay can help.
Big thing.....get books! Read as much as you can on horses.....care, training, etc. Buy them, go to the library, borrow them from others. Educate yourself as much as possible & ask questions....even if they seem stupid! If you don't know the answer then it isnt a stupid question.
Post some pics if you can and......
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