Sometimes teenagers can be terribly cruel. Often there is no objective reason why someone becomes the object of bullying, but once it happens it’s very hard to stop it. The best news in your letter is that your brother has graduated from high school and will be starting University. This gives him a chance for a fresh start with new people. No one at University will know that he was a target. No one at University will think of him in any particular way until he starts interacting with them.
Your brother is a lucky man indeed to have such a caring sister. Yes, there are some things you can do to help. I hope you have the kind of relationship where you can talk frankly and sympathetically to him about making a new start. Remind him that he can be whoever he wants to be at his new school. I actually know some young people who changed their names when they went to college as a way to separate themselves from painful pasts. By changing their names, they were helping themselves change themselves. Talk to your brother about how lying and exaggerating will invite the bullying to start again. See if he will let you have a conversation about how to enter into new relationships in a new way. Don’t shame him or scold him. Be loving and supportive and perhaps he’ll be able to respond. Even if he can’t talk about it, you will have given him something useful to think about.
One more thing: If he is receptive, you might tell him for me that one of the best ways to adjust to a new school is to find a club or organization that interests him and get active in it. People like people who are interested in the same things and who are willing to participate and contribute. By focusing on how to be helpful, he will take some of the pressure off himself about making friends right away. It’s often when people focus on a mutual goal instead of on making friends that the best relationships happen.
I wish you well.