Wow, this has so many responses...
Please take the time to read this to the end...
Sometimes when a teenager lets themselves go, it is a rebellion against those they really want to be like. Because they feel they will never make it on that social level, so they rebel against it instead of risking failure. Perfection is such an unobtainable goal.
Instead of accepting himself as an individual, he sees his differences as a flaw. People around him put forth such a perfect social image that he doesnt see their flaws...there is no such thing as a perfect person, but he feels imperfect so he builds a more imperfect image to show his disregard for social acceptance....
So, maybe it's all rebellion.
Here is some silly things to think about...
Does he show any resentment for the 7 yr old...perhaps feeling like he was given a better hand in life?
Does he have a lot of friends who are like him just part of a rebellious group or is he a bit alone?
Does he consider himself to be dumb compared to others?
Pay attention to him and learn from him by observation, because he will not tell you whats going on in his head.
Sometimes they stop caring for their appearance and status because they feel it is hopeless to strive for it because will never be recognized. One day he may be like a switch went on and suddenly find who he wants to be and will accept himself, cherish his differences and be confident in himself because of his individuality instead of withdrawing from the world.
He may be rebelling against those with stable households and structured lives. Although a custody battle may help him by putting him where he may prosper, it may also crumble his life by adding more instability. And, the worst part.... You are pointing out that he is not perfect to you. Don't try to change him. But, you can simply educate him on responses to his behavior and image. Let him know whatever he chooses is his own thing and as family you will always accept him....but he will be subject to stereotyping by outsiders (which he may not care about).
Ok, fo instance, when you go to a mall, if you look like a social deviant, you are treated like a deviant. Guards follow you, and sales people do not want to help you. When you need help in life, it is easier to get help when you appear harmless. But when you look scary, people will lock their car doors and speed away. I took my daughter to the mall wearing hospital scrubs....LOL... I was wearing flip flops and scrubs...the whole outfit was worth under $25....but I was treated like a working class citizen. Now if I wore a $25 dollar outfit from the goodwill and looked dirty and unkept I would've been treated differently. A $25 outfit new and clean from walmart, would be treated differently yet. Educate him, but do not judge or try to change him. If he chooses to have a grunge appearance despite knowing the consequence, who are you to judge him? He may not care that he won't be invited to be a member of the social elite; he may have better things deep down to focus on. But, he needs to understand that someone out in the world is seeing him as a deviant and a rebel....and he may have to sit in front of them in a job interview one day. He needs to earn respect from the world.
There are other responses to your request I read about screening for depression in youth. Depression is very real in todays youth. But it doesn't always need to be medicinally treated. Depression from chemical imbalance occuring for no reason, should be treated. Depression caused by emotional stress....should be dealt with emotionally, not chemically. Fix the emotional stress and they can balance the chemistry on ther own. Masking it with antidepressants does not help them to find a cure for their unhappiness. Nobody can make somebody be happy. Do not rush for medicine as an answer!! Besides, by giving him meds, you may be stunting his growth in other ways that perhaps you are not seeing yet....stunting his individuality and/or artistic growth.
I am respected in life as being a voice of reason and full of answers. Most people admit I am the most intelligent person they know. But as a youth, I was a substandard student, unpopular, withdrawn and rebellious. But I was very artistic and had better things to focus on....socialization came later in life. Albert Einstien was a terrible student, but his mind was developing in ways far more important that being socially acceptable. When a toddler walks early, it does not indicate that it will be an athlete....when they talk late, it doesn't mean they ae dumb. He is still developing as an adolescent....let him go at his pace....but keep providing good examples.
The best thing you can do, is care. If you are too forceful in trying to change him, you may make him feel inferior or substandard. Love unconditionally....Even if you don't think he is responding, you are still teaching him what a responsible adult / parent should be like. You don't know what his home life is like, but you can show an example of a healthy happy home by accepting him for who he is. He is family and you will always love him whether he has good grades or bad grades....is popular or not...and whether or not he has a socially acceptable image. All you can hope for is that he views your household as a goal. The way he wants to be as an adult. But, it is up to him to decide when to find that path and when to work for it.
But most importantly...you are teaching him - that to truly love someone, is to love an imperfect person.