yeah i've also got a person who is two years younger than me and is all ways ringing me up to have pointless conversations! and belive me your not selfish for not wanting to go out with him.
First of all you say hes not mentally alright what do you mean by this?
The reason he wants to hang out with you guys is probally because he sees you as cool and wants to be like you! maybee he doesn't have an elder brother of his own. Or he could just be lonely and not have many friends. In any case don't get angry at him because he could be having a really bad time.
I would ask him is he has any friends his own age and if he dosn't ask him why it may be that he has like social problems or something.
Love me for what I am hate me for what I am not