Mate, dont end up looking after him. Like you said yourself, you are a teenager. It's not that you can't look after him, its that you shouldn't have to. You have your own life to lead.

I would 100% get in touch with the people at the mental health centre if he seems to get worse. Otherwise, leave him to it. Hell, he might do better and keep his self esteem up if he pulls himself out of it. But keeping an eye on him is probably a good idea.

I hope you feel loads better soon! Stomach flu is the pits. ):