Where did he get an idea like that? :shifty:
I am stupid to think that our relationship will get better when we grow up.
Not necessarily. I get on much better with my sister now that we're grown up ... not that we really got on badly as kids, though.
he has asked me to do him favor to put seal on documents that proves he has been an intership in a company.
Simple solution: never do anything for anyone that's dodgy, for any reason. (I assume this is something less than honest?) It doesn't help anyone. Set an example of honesty and uprightness. He may come to respect you for it later on. Simply be ready to get on well with him if he acts with dignity. Just be an example of uprightness and good dealings, and let him choose whether to be on good terms with you or not.