I was 5'9" 170 when I was 13. I was heavy into football and weightlifting, but I also didn't have any guidance at home or emotional support from my parents. My love for football and lifting couldn't stop the downward spiral.
It sounds like he's in the same boat as I was in, except he has a father who loves him and wants to help. I don't think there is a single more powerful influence in a boy's life than his father. Especially at that age.
Unfortunately, the world is a lot different than it was when you were growing up. This new global community we have online really prevents kids from getting a sense of what's real and what's fluffed up bullshit. Everybody wants to be like the people they see on social media. Kids don't realize they only see the highlight reel.
I think it's pretty simple: support the positive things he wants to do. If he wants to play football and work out, then build both of your lives around sharing that journey together. The more he begins to believe in himself via the belief you have in him, the more it'll drive him toward bettering himself. Teach him what happens to the kids who start slacking off in school and getting high. Teach him about delayed gratification. Teach him to be responsible for himself while supporting him every step of the way.
I'm not a father, but I know that if I would have a dad who did these things for me, I wouldn't have had to learn how to do it for myself in my mid-twenties after fucking my life up. Good luck.