I'm really doubting the ages of you all...this does not seem like it is written by a 20 year old...and the issue about tattle-taling doesn't sound like it's between a 20 year old and a 15 year old...because 20 year olds are typically too old to tattle tale and the language just seems off for a 20 year old and a teenage boy typically doesn't "hide".

But maybe his resentment has nothing to do with you. Maybe there is a bigger issue you don't know about and he's struggling to cope with it. High school can be rough for teenagers. Maybe he's sad you're leaving for college and he's not going to see you for a long time. I'd suggest be a grown up about it and have a talk with him.